French Onion Smash Burger with Gruyère & Aioli

This French Onion Smash Burger brings the deep, sweet flavor of classic French onion soup to the ultimate crispy-edged smash burger.

Caramelized onions are cooked low and slow until jammy, then piled high over two thin, lacy beef patties and blanketed with aged Gruyère. Toasted brioche and a sharp Dijon aioli tie it all together. It's a rich, savory burger that feels both familiar and exciting.

The secret to this burger is layering textures and flavors: the sweet, melt-in-your-mouth onions, the salty, crispy beef edges, the nutty melted Gruyère, and the tangy aioli.

Every bite is a contrast of crunchy and soft, savory and sweet. While the onions take a little time, the hands-on work is quick. This recipe turns a weeknight dinner into something special without any complicated techniques.

Ingredients

Why this recipe works

Smashing the beef into thin patties maximizes the surface area, creating those coveted crispy, lacy edges that are the hallmark of a great smash burger.

The high heat of a cast iron skillet ensures a deep sear that adds a savory crust. The 80/20 ground beef provides enough fat to keep the patties juicy and flavorful, even when cooked to a well-done temperature.

Caramelizing the onions slowly in butter draws out their natural sugars, turning them into a sweet, jammy condiment that echoes the classic French onion soup topping. The aged Gruyère melts into a rich, nutty layer that complements the sweet onions and savory beef. The Dijon aioli cuts through the richness with a bright, tangy kick, balancing every bite.

Notes and substitutions

Use a high-quality ground beef with 20% fat for the best flavor and juiciness. The fat is essential for the crispy edges and prevents the patties from drying out. If the beef is too lean, the burgers will be tough and less flavorful. Chilling the beef balls before smashing helps them hold together and sear properly.

Yellow onions are ideal for caramelizing because they are sweet and cook down beautifully. Be patient and cook them over low heat — rushing will burn them.

A cast iron skillet is the best choice for smashing burgers because it retains heat well and gives a great crust. A sturdy metal spatula is crucial for smashing and flipping the patties without breaking them.

• brioche buns: Potato rolls or soft hamburger buns work well; toast them the same way.
• aged Gruyère: Swiss cheese or Emmental can replace Gruyère for a similar nutty melt.
• ground beef (80/20): Ground chuck with 20% fat is ideal; leaner beef will be less juicy.
• Dijon mustard: Whole-grain mustard adds a different texture but similar tang.

Equipment

• Cast iron skillet or griddle
• Metal spatula
• Basting dome or large metal bowl
• Mixing bowl
• Whisk
• Chef's knife
• Cutting board