French Onion Smash Burger with Gruyère & Aioli Step-by-step instructions

Before you start

Before cooking, make sure the ground beef is well chilled. Slice the onions thinly and have all ingredients measured and ready. The aioli can be made ahead and stored in the refrigerator.

Preheat the cast iron skillet over high heat for at least 5 minutes so it's ripping hot when the beef hits the surface. Have the brioche buns split and ready to toast.

Instructions

Caramelizing and Aioli

1. 1Caramelize the Onions
   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.
   Look for Onions are dark mahogany, soft, and glossy, with a sweet aroma.
   Tip Low heat is key — rushing will burn the onions before they caramelize.
2. 2Make the Dijon Aioli
   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.
   Look for The aioli is creamy, pale yellow, and well blended.
   Tip Chilling the aioli lets the flavors meld while you work.

The onions should be deep brown, soft, and sweet, with a jammy texture. The aioli should be smooth and well combined. The kitchen will smell of sweet butter and onions. The onions should be fully caramelized before moving on to the next phase — they can be kept warm while the burgers cook.

Smashing and Melting

3. 3Smash the Beef
   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.
   Look for Patties are thin with craggy, irregular edges that will crisp up.
   Safe temperature 160°F (71°C) internal temperature (ground meat)
   Tip Smash hard and fast — the thinner the patty, the lacy the edges.
4. 4Cook Until Crispy
   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).
   Look for The undersides are golden brown with crisp, lacy edges.
   Safe temperature 160°F (71°C) internal temperature (ground meat)
   Tip Resist the urge to move the patties — letting them sear creates the crust.
5. 5Top with Onions and Cheese
   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.
   Look for Cheese melts into a gooey, golden blanket over the onions.
   Safe temperature 160°F (71°C) internal temperature (ground meat)
   Tip The dome helps the cheese melt without overcooking the beef.
6. 6Toast the Brioche
   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.
   Look for Buns are golden with a light crunch on the cut side.
   Tip Toasting the buns adds texture and prevents sogginess.

The beef patties should be thin with crispy, lacy edges. The cheese should be melted and bubbling over the onions. The buns should be toasted and golden. The skillet should be hot enough that the beef sizzles immediately on contact. The patties should reach 160°F (71°C) internally.

Assembling and Serving

7. 7Assemble the Burgers
   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.
   Look for The burger is stacked high with melted cheese and onions.
   Tip Press down gently to help the burger hold together.
8. 8Serve Immediately
   Serve the burgers hot, with extra aioli on the side if desired. Press down slightly to let the juices and melted cheese mingle.
   Look for Juices and cheese ooze out as the burger is pressed.
   Tip Serve right away for the best texture and flavor.

The burgers are stacked high with aioli, patties, onions, and cheese. The buns should be toasted and sturdy enough to hold the filling.

The burger should be served immediately, with juices and cheese oozing out when pressed. The final burger should look glossy and inviting, with visible crispy edges and melted cheese.

Common mistakes

One common mistake is cooking the onions too quickly over high heat, which burns them before they caramelize. Keep the heat low and be patient. Another mistake is not letting the skillet get hot enough before adding the beef — if the pan is not smoking, the patties will steam instead of sear.

Also, avoid pressing the patties after they are smashed, as this squeezes out the juices. Finally, don't skip toasting the buns; it adds essential texture.

How to tell it’s done

The burgers are done when the internal temperature of the patties reaches 160°F (71°C) on an instant-read thermometer. The onions should be deeply caramelized, soft, and jammy, with a rich sweet aroma.

The cheese should be fully melted and bubbling. The brioche buns should be golden brown and crisp on the cut side. The burger should be hot throughout and the beef should be cooked through with no pink remaining.

• Ground meat: 160°F (71°C) in the thickest part