If you’re craving something savory, rich, and downright irresistible, let me introduce you to my Buttery Roast Beef Sliders with Caramelized Onions Recipe. These little handheld delights turn ordinary roast beef sandwiches into a gourmet experience packed with melty cheeses and that luscious garlic butter kick.

Two toasted sandwich buns with a golden brown color and sprinkled with sesame seeds and seasoning sit stacked on a dark wooden board. The bottom bun holds a layer of melted cheese that drapes over the edges, topped with caramelized onions and slices of cooked meat peeking out from under the cheese. The top bun rests slightly off-center on the filling, showing more melted cheese and caramelized onions oozing from the sides. The background is dark, and the board contrasts with the textures of soft buns, shiny melted cheese, and juicy meat and onions. Photo taken with an iphone --ar 2:3 --v 7

Jump to:

Why You'll Love This Recipe
Ingredients & Why They Work
Make It Your Way
Step-by-Step: How I Make Buttery Roast Beef Sliders with Caramelized Onions Recipe
Top Tip
How to Serve Buttery Roast Beef Sliders with Caramelized Onions Recipe
Make Ahead and Storage
Frequently Asked Questions:
Final Thoughts
Buttery Roast Beef Sliders with Caramelized Onions Recipe

Why You'll Love This Recipe

I honestly can’t get enough of these sliders. Every time I make them, friends ask for seconds (and thirds!). The combination of tender roast beef, sweet caramelized onions, and that dreamy garlic aioli just hits the spot every single time.

Decadent Flavor: Each slider is bursting with rich, buttery goodness and savory roast beef layers that melt in your mouth.
Perfect Party Food: These sliders are crowd-pleasers that make entertaining stress-free and fun.
Make-Ahead Friendly: You can prepare components in advance, then assemble and bake when ready.
Comfort Food with a Twist: Classic roast beef sandwiches get elevated with gourmet touches like garlic butter and homemade au jus.

Ingredients & Why They Work

Before you dive in, it’s helpful to gather a few key ingredients that really make these sliders shine. Picking fresh onions and good-quality roast beef will up your game—and Hawaiian slider buns are my top pick for that soft, sweet base that soaks up all the buttery sauce just right.

Flat lay of a stack of 3 large yellow onions, thinly sliced; a small white ceramic bowl with a smooth pale yellow egg yolk; a single whole garlic clove, white and firm; a small white ceramic bowl filled with light golden olive oil; a small white ceramic bowl with bright yellow fresh lemon juice; a pat of unsalted butter, pale creamy and rectangular; a small white ceramic bowl of brown sugar; a small white ceramic bowl of coarse kosher salt; a small white ceramic bowl of deep brown beef broth; a small white ceramic bowl of dark Worcestershire sauce; a handful of thinly sliced deli roast beef, deep red with marbled fat; twelve soft, round slider buns with a glossy golden-brown crust sliced in half; twelve smooth, pale provolone cheese slices; a small white ceramic bowl of shredded white mozzarella cheese; a small white ceramic bowl with poppy seeds; a small white ceramic bowl with white sesame seeds; a small white ceramic bowl with dried minced onion; a small white ceramic bowl with garlic powder; a few whole brown eggs with clean shells arranged symmetrically nearby, placed on a clean white marble surface, soft natural light, photo taken with an iPhone, professional food photography style, fresh ingredients, white ceramic bowls, no bottles, no duplicates, no utensils, no packaging --ar 2:3 --v 7 --p m7354615311229779997

Unsalted butter: Essential for caramelizing onions, making garlic butter, and the au jus, giving everything a rich, silky texture.
Olive oil: Helps cook the onions gently without burning and adds a subtle fruity undertone.
Yellow onions: Their natural sweetness shines through after slow caramelization, balancing the savory beef perfectly.
Kosher salt: Used to season every stage for depth and a clean, bright flavor.
Brown sugar: An optional catalyst that speeds up onion caramelization and enhances sweetness.
Beef broth: Adds savory depth to the onions and au jus sauce for that authentic roast beef sandwich vibe.
Egg yolk: The creamy base for homemade garlic aioli that adds tang and richness.
Dijon mustard: Brings a mild kick and emulsifying power to the aioli.
Garlic: Freshly grated for the aioli and garlic butter, providing that irresistible aromatic punch.
Lemon juice: Balances the aioli with a bright acidic note.
Neutral or light olive oil: Allows the aioli to thicken smoothly without overpowering flavors.
Better than Bouillon Beef Base: Concentrated, savory beef flavor for a rich au jus.
Worcestershire sauce: Adds umami complexity and depth to both the au jus and garlic butter sauce.
Slider buns (Hawaiian rolls preferred): Soft, sweet, and just the right size to hold all the fillings without falling apart.
Deli roast beef: Thinly sliced for tender layers that don’t overwhelm the bun or other components.
Melty cheese slices: Provolone, Swiss, or muenster work beautifully to add gooey richness.
Shredded mozzarella cheese: Adds extra ooey-gooey melt on top of the slices.
Poppy seeds and white sesame seeds: Sprinkled on the garlic butter tops for crunch, texture, and flavor.
Garlic powder and dried minced onion: Boost the garlic butter sauce with concentrated savory notes.
Freshly cracked black pepper: Adds a little heat and complexity to the butter sauce seasoning.

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Note: You'll find the complete list of ingredients, along with their exact measurements, in the printable recipe card at the bottom of this post.

Make It Your Way

One of the best parts about the Buttery Roast Beef Sliders with Caramelized Onions Recipe is how easily you can personalize it to suit your taste or occasion. Whether you’re short on time or looking to add a special twist, these sliders lend themselves beautifully to tweaks and tweaks that make the recipe your own.

Speedy Slider Swap: If you're pressed for time, try skipping the caramelized onions altogether and jazz up mayo with the garlic aioli ingredients for a quick, flavorful spread. These sliders bake up beautifully in about 30 minutes and still deliver plenty of punch without the onion slow-cook.
Cheese Choices: Feel free to experiment with different melty cheeses. I personally love a mix of provolone with shredded mozzarella, but Swiss or muenster also work wonders for that gooey, melty goodness.
Make-Ahead Magic: For gatherings, prep the sliders up to two days ahead but hold off on adding the caramelized onions and garlic butter. Assemble and bake right before serving to keep everything fresh and the buns perfectly soft yet toasty.
Diet Adjustments: For a lighter take, try swapping half the butter in the garlic butter sauce with olive oil or use a lower-fat cheese. You can also experiment with gluten-free slider buns to accommodate dietary preferences.
Seasonal Twists: Add a touch of horseradish cream or a smear of whole grain mustard for a zesty seasonal update to your sandwiches that pairs amazingly with the savory roast beef and sweet onions.

Step-by-Step: How I Make Buttery Roast Beef Sliders with Caramelized Onions Recipe

A close-up shows a woman's hand holding a sandwich with three layers: a toasted, sesame and poppy seed covered top bun in golden brown with a shiny glaze; inside there are layers of caramelized onions in dark brown with a glossy texture, melted cheese dripping in light yellow, and thin slices of cooked meat in pink with browned edges; below is the bottom bun matching the top in texture and color. The sandwich is being dipped into a white bowl filled with thick, brown dipping sauce that is dripping down the sides and splashing on the wooden surface below. In the background, there are more stacked sandwiches with the same layers on a wooden board, set against a white marbled textured surface. Photo taken with an iphone --ar 2:3 --v 7

Step 1: Slowly Caramelize Onions to Deep Golden Perfection

Start by melting 2 tablespoons of unsalted butter and 1 tablespoon of olive oil in a large skillet over medium-low heat. Add your thinly sliced yellow onions, sprinkle with ½ teaspoon kosher salt and 1 teaspoon brown sugar to speed up caramelization. Let them cook slowly, stirring every few minutes to avoid burning, for about 45 minutes until they turn soft, jammy, and a deep golden brown. The slow cooking really brings out their natural sweetness and rich flavor. When ready, stir in 2 tablespoons of beef broth to deglaze the pan and scoop up all those delicious browned bits. Set the onions aside and savor that amazing aroma — it’s the heart of these sliders!

Step 2: Whip Up a Lusciously Creamy Garlic Aioli

In a small bowl, whisk together 1 large egg yolk (room temperature), 1 teaspoon Dijon mustard, 1 finely grated garlic clove, and 1 tablespoon fresh lemon juice until smooth. Now, while whisking constantly or using an immersion blender, slowly drizzle in ½ cup of light olive oil until the mixture thickens into a rich, creamy aioli. Season with kosher salt to taste. If you want to skip the emulsification, simply mix these ingredients into mayonnaise for a quicker, still flavorful alternative. This garlicky spread adds a fresh, tangy punch that perfectly balances the richness of the roast beef and cheese.

Step 3: Simmer a Savory Au Jus Sauce

Melt ¼ cup unsalted butter in a medium saucepan over medium heat. Stir in 1 tablespoon Better than Bouillon Beef Base, 2 cups beef broth, and 2 teaspoons Worcestershire sauce. Let everything gently simmer for 20 minutes, reducing by about half. The sauce will darken slightly and thicken as it cools, becoming the perfect warm dip for the sliders. Keep it on low heat to stay warm for serving.

Step 4: Toast Buns for a Buttery Base

Preheat your oven to 350°F (180°C). Slice your 12 slider buns in half horizontally, but keep the pairs together so they hold nicely on the baking sheet. Place the bottom halves cut-side up on parchment paper. Brush lightly with either melted butter or garlic aioli for extra flavor and bake for 5 minutes until just golden and slightly crisp. This subtle toasting creates a wonderful base that holds up against all the juicy goodness coming next.

Step 5: Layer Up the Goodness

Begin assembling by spreading a generous layer of your homemade garlic aioli on each toasted bottom bun. Pile on ½ pound of thinly sliced deli roast beef, arranging it in 2 to 3 hearty layers for that perfect slider heft. Top each slider with 12 slices of your chosen melty cheese, followed by ½ cup shredded mozzarella to boost that melty, gooey texture. Finally, spoon 2 cups of those luscious caramelized onions on top, adding sweetness and depth in every bite.

Step 6: Garlic Butter Topping and Baking

Top each slider with the remaining bun halves. In a small bowl, whisk together 6 tablespoons melted unsalted butter, 1 teaspoon Worcestershire sauce, 1 tablespoon poppy seeds, 1 tablespoon white sesame seeds, ½ tablespoon garlic powder, ½ tablespoon dried minced onion, ½ teaspoon kosher salt, and freshly cracked black pepper to taste. Generously brush this savory garlic butter mixture on top of the slider buns. Cover the pan loosely with foil and bake for 10 minutes. Then remove the foil and continue baking for another 10 minutes until the buns are golden brown and the cheese is melted and bubbly.

Step 7: Serve Warm with Au Jus Dipping

Once baked, carefully slice between the buns to separate your sliders. Serve them warm alongside small bowls of the rich, savory au jus sauce for dipping. This little step takes each bite over the top, giving your sliders a juicy, tender finish that’s absolutely irresistible. Perfect for parties, game days, or any occasion that calls for something special!

Top Tip

Mastering these Buttery Roast Beef Sliders with Caramelized Onions Recipe is all about layering flavors and textures just right. Here are some tips I’ve picked up to make sure your sliders come out perfect every time!

Caramelizing Onions Low and Slow: Patience truly pays off here. Cooking your onions over medium-low heat for about 45 minutes transforms them into velvety, sweet, jammy perfection—and it’s a total game-changer for flavor depth.
Garlic Aioli Magic: If you’re short on time, mixing aioli ingredients into mayo is a brilliant shortcut. It speeds things up without sacrificing that lovely garlicky tang I crave.
Brushing the Buns Just Right: I like to brush the slider tops generously with the garlic butter mixture—that combo of Worcestershire, poppy seeds, and sesame seeds really brings the buns to life and adds incredible aroma and crunch.
Avoid Soggy Sliders: Toast the bottom buns before assembly and reheat leftovers in the oven at 375°F instead of the microwave. It keeps everything crisp and melty without turning soggy.

How to Serve Buttery Roast Beef Sliders with Caramelized Onions Recipe

The image shows five pieces of bread spread with a creamy white mixture that has small green flecks, all placed neatly in a baking tray. The bread pieces vary in size and shape, with three longer slices arranged horizontally on the top and middle, and two smaller slices on the lower right side. The creamy spread is unevenly applied, with some areas thicker and others thinner, showing a rough texture on the bread underneath. The baking tray has a light beige tone, and the background surface has been changed to a white marbled texture. Photo taken with an iphone --ar 2:3 --v 7

Garnishes

To elevate these sliders visually and flavor-wise, consider garnishing with fresh sprigs of thyme or parsley for a pop of color and herbaceous brightness. A few crunchy dill pickle slices on the side add a delightful contrast, and a sprinkle of flaky sea salt on top of the garlic butter brush just before baking enhances that buttery richness.

Side Dishes

These sliders pair beautifully with classic sides like crispy shoestring fries, tangy coleslaw, or a fresh garden salad to balance the richness. For a cozier meal, creamy potato gratin or roasted garlic mashed potatoes work wonderfully. And don’t forget to have extra bowls of the homemade au jus for dipping—it’s pure comfort in every bite!

Make Ahead and Storage

Storing Leftovers

Once your sliders have cooled, store any leftovers in an airtight container in the refrigerator for up to 3 days. Keeping them sealed well helps maintain the soft bun texture and rich flavor without drying out the roast beef or cheeses.

Freezing

You can freeze the fully assembled sliders before baking if you want to prep ahead for busy days or parties. Wrap them tightly in plastic wrap and foil to protect flavor and moisture. When ready to enjoy, bake from frozen—just add a few extra minutes to the baking time and cover loosely with foil at first to prevent over-browning.

Reheating

To reheat leftovers without losing that fresh-baked charm, preheat your oven to 375°F. Place the sliders on a baking sheet and warm for about 10 minutes until the cheese is melty and the tops regain their buttery crispness. Avoid microwaving to keep the buns from getting soggy.

Frequently Asked Questions:

Can I use store-bought caramelized onions to save time?

Absolutely! While homemade caramelized onions add the best depth of flavor, store-bought caramelized onions can work in a pinch. Just heat them gently before adding so they blend well with the roast beef and cheese.

What’s the best cheese for these sliders?

Melty cheeses like provolone, Swiss, or muenster are ideal because they melt nicely and complement the roast beef without overpowering. The shredded mozzarella adds a gooey, cheesy texture that everyone loves!

Can I make these sliders ahead of time?

Yes, you can assemble the sliders up to two days in advance without the caramelized onions or garlic butter. Then add those just before baking for the freshest flavor and best texture.

How long do these sliders keep after baking?

Store any leftovers in an airtight container in the fridge for up to 3 days. Reheat gently in the oven at 375°F to preserve the slider’s texture and flavor.

Final Thoughts

Making these Buttery Roast Beef Sliders with Caramelized Onions has become one of my favorite go-to recipes for gatherings and cozy dinners alike. There’s something so comforting about biting into that warm, buttery bun layered with tender roast beef, gooey cheese, and those sweet, caramelized onions. I hope this recipe brings as much joy and deliciousness to your table as it has to mine. Happy cooking and even happier eating!