French Onion Skillet Smashed Burgers with Melted Gruyère Step-by-step instructions

Before you start

Before starting, slice the onions thinly and measure out the butter and broth. Divide the ground beef into four loose balls and refrigerate them until ready to cook.

Set out the Gruyère slices and Dijon aioli so everything is within reach. Heat the cast-iron skillet over medium-low for the onions, then increase to high for the smash step.

Instructions

Caramelize the Onions

1. 1Caramelize the Onions
   Melt the butter in a large cast-iron skillet over medium-low heat. Add the sliced onions and cook slowly, stirring occasionally, for 25–30 minutes until they are deep golden brown and jammy. Add the beef broth and continue cooking for 2–3 minutes, scraping up any browned bits. Season with salt and pepper to taste. Transfer the onions to a bowl and wipe the skillet clean.
   Look for Onions are soft, dark brown, and sweet, with a jam-like texture.
   Tip Keep the heat low to avoid burning; add a splash of water if they stick.
2. 2Prepare the Beef
   Divide the ground beef into 4 equal portions (about 4 oz each). Shape each portion into a loose ball without overworking the meat. Keep the balls chilled until ready to cook.
   Look for Meatballs are loosely packed and cold.
   Tip Chilling helps the patties stay juicy and prevents crumbling.

The onions should be deeply browned, soft, and sweet, with a jam-like consistency. The butter and broth should have reduced into a glossy glaze. The skillet should be wiped clean for the next step.

Smash and Sear

3. 3Smash the Patties
   Heat the same cast-iron skillet over high heat until it is smoking hot. Place one beef ball in the skillet and immediately smash it down with a spatula until it is very thin, about 1/4 inch thick. Season generously with salt and pepper. Cook for 2–3 minutes without moving until the edges are browned and crispy.
   Look for Patties have lacy, crispy edges and release easily from the pan.
   Safe temperature 160°F (71°C) internal temperature (ground meat)
   Tip Use a heavy spatula and press firmly; do not move the patty while it sears.
4. 4Flip and Add Onions and Cheese
   Flip the patties and immediately pile a generous amount of caramelized onions on top of each patty. Lay slices of Gruyère cheese over the onions. Cover the skillet with a lid or dome and cook for 1–2 minutes until the cheese is melted and bubbly.
   Look for Cheese is fully melted and starting to bubble.
   Safe temperature 160°F (71°C) internal temperature (ground meat)
   Tip Covering traps steam and melts the cheese quickly without overcooking the beef.

The patties should be thin with lacy, crispy edges and a deep brown crust. The cheese should be fully melted and bubbling over the onions. The kitchen will smell of toasted beef and caramelized onions.

Assemble and Serve

5. 5Toast the Buns
   While the patties finish, toast the brioche buns cut-side down in a dry skillet or under a broiler until golden brown.
   Look for Buns are golden and lightly crisp on the cut side.
   Tip Toast just before serving to keep them warm and firm.
6. 6Assemble the Burgers
   Spread the Dijon aioli on the bottom half of each toasted bun. Place one cheesy patty on each bottom bun, then top with the second patty (if using double) or additional onions. Close with the top bun and press down gently.
   Look for Burgers are stacked tall with melted cheese and onions.
   Tip Press gently to meld the flavors without squeezing out the juices.

The buns should be toasted and golden, the aioli spread evenly, and the burgers stacked high with melted cheese and onions.

Pressing down gently should release a little juice, but not so much that the bun becomes soggy.

Common mistakes

Common pitfalls include cooking the onions over too high heat, which burns them before they soften; overcrowding the skillet, which steams the patties instead of searing; and moving the patties too soon, which tears the crust.

Also, avoid overworking the beef, which makes the patties tough.

How to tell it’s done

The patties are done when they reach an internal temperature of 160°F (71°C) and the edges are deeply browned and crispy.

The cheese should be fully melted and bubbling, and the onions should be dark golden and jammy. The buns should be toasted and golden, and the aioli adds a tangy contrast.

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