French Onion Smash Burger with Gruyère & Aioli Tips, FAQ & printable recipe

Tips for success

For the crispiest edges, make sure the beef is very cold before smashing. Use a heavy spatula and press down firmly and quickly to flatten the patty in one motion.

Don't move the patties once they hit the pan — let them sear undisturbed to develop that deep brown crust. If the pan is crowded, cook the patties in batches to maintain high heat.

When caramelizing onions, a pinch of salt helps draw out moisture and speed up the process. If the onions start to stick, add a splash of water or broth to deglaze the pan.

The onions can be made ahead and reheated gently. For the aioli, use a good-quality mayonnaise and adjust the Dijon to taste. Let the aioli sit for a few minutes to allow the garlic flavor to mellow.

Variations

• Mushroom Swiss: Sauté sliced mushrooms with the onions and use Swiss cheese instead of Gruyère. Add a spoonful of Dijon aioli for a similar tangy finish.
• Spicy Kick: Add a few dashes of hot sauce to the aioli or top the burger with sliced jalapeños. Use pepper jack cheese instead of Gruyère for extra heat.
• Bacon Lover's: Add crispy bacon strips to the burger before the cheese melts. The smoky flavor pairs well with the sweet onions and nutty Gruyère.
• Classic French: Use a slice of toasted baguette instead of brioche for a crustier bite. Replace the aioli with a simple butter and Dijon spread.

Storage and make-ahead

Fridge

Store leftover burgers in an airtight container in the refrigerator for up to 3 days.

Freezer

Freeze cooked burgers (without buns) for up to 2 months. Wrap individually in plastic wrap and place in a freezer bag.

Reheat

Reheat frozen burgers in a 350°F (175°C) oven for 10-12 minutes, or until hot. Reheat buns separately. The onions and aioli can be used cold or gently warmed.

Store leftover burgers in an airtight container in the refrigerator for up to 3 days. Reheat in a skillet over medium heat to refresh the crispy edges, or in the microwave for a quick meal. The onions and aioli can be stored separately for up to 5 days.

The caramelized onions can be made up to 3 days ahead and stored in the refrigerator. The Dijon aioli can also be prepared in advance and kept chilled. When ready to cook, bring the beef to room temperature for 30 minutes before smashing, then proceed with the recipe.

Serving ideas

Serve the French Onion Smash Burgers hot, straight from the skillet. Pair them with crispy fries, a simple green salad, or pickles to cut through the richness.

The burger is substantial enough to be a meal on its own, but the tangy aioli and sweet onions balance the savory beef.

For a complete French onion soup experience, serve the burger with a side of roasted vegetables or a light broth.

The burger also works well as a game-day or casual dinner option. Let the burger rest for a minute after assembling to allow the cheese to set slightly, making it easier to eat.

Frequently asked questions

Recipe card

French Onion Smash Burger with Gruyère & Aioli

Prep 15 minCook 35 minTotal 50 minServes 2

Ingredients

• 2 balls ground beef (80/20), 100g each
• 2 medium yellow onions, thinly sliced
• 2 tbsp unsalted butter
• Kosher salt, to taste
• Freshly ground black pepper, to taste
• 4 slices aged Gruyère cheese
• 2 brioche buns, split
• 1/2 cup mayonnaise
• 1 tbsp Dijon mustard
• 1 clove garlic, minced

Instructions

1. Melt the butter in a large cast iron skillet over medium-low heat. Add the sliced onions and a pinch of salt. Cook slowly, stirring occasionally, for 20 to 25 minutes, until the onions are deep brown, jammy, and very sweet. If they start to stick, add a splash of water.
2. In a small bowl, whisk together the mayonnaise, Dijon mustard, and minced garlic until smooth. Set aside in the refrigerator while you cook the burgers.
3. Heat a cast iron skillet or griddle over high heat until smoking. Place the chilled beef balls onto the hot surface. Using a sturdy metal spatula, press each ball down firmly to flatten into thin patties, about 1/4 inch thick. Season generously with salt and pepper.
4. Cook the patties without moving them for 2 to 3 minutes, until the edges are deeply browned and crispy. Flip the patties and cook for another 1 to 2 minutes on the second side, until the internal temperature reaches 160°F (71°C).
5. After flipping, pile a generous mound of caramelized onions onto each patty. Lay two slices of Gruyère cheese over the onions. Cover the skillet with a basting dome or a large metal bowl to trap the heat and melt the cheese.
6. While the cheese melts, place the split brioche buns cut-side down in a separate dry skillet over medium heat. Toast for 1 to 2 minutes, until golden brown and lightly crisp.
7. Spread a generous layer of Dijon aioli on the cut side of each toasted brioche bun. Place one cheesy patty on each bottom bun. Top with the other patty if stacking double, then close with the top bun.
8. Serve the burgers hot, with extra aioli on the side if desired. Press down slightly to let the juices and melted cheese mingle.

Contains: milk, eggs, gluten, mustard

Estimated nutrition per serving

Calories

780

Protein

38 g

Carbohydrates

42 g

Fat

52 g

Automatic estimate from the ingredient list, not laboratory-verified.