Smashburgers with Special Truffle Sauce

Ingredients

• 1/2 cup mayonnaise
• 1/4 cup finely chopped cornichons
• 1 tablespoon Dijon mustard
• 1 tablespoon freshly squeezed lemon juice
• 1 tablespoon black truffle oil
• Kosher salt and freshly ground black pepper, (to taste)
• 1-2 tablespoons extra-virgin olive oil
• 2 yellow onions, (thinly sliced)
• Pinch of kosher salt
• 2 pounds ground beef (85/15)
• Kosher salt and freshly ground black pepper, (to taste)
• unsalted butter (optional)
• 6 brioche buns, (sliced)
• 12 slices Gruyère, (Swiss, or white cheddar cheese)
• Baby arugula
• Cornichons

Instructions

1. Make the Truffle Sauce:
2. In a small bowl, combine the mayonnaise, cornichons, mustard, lemon juice, and truffle oil. Season with salt and pepper to taste. Cover and refrigerate until ready to use.
3. Prepare the Caramelized Onions:
4. Heat 1 to 2 tablespoons oil in a large skillet set over medium heat. Add the onions and a pinch of salt. Cook, stirring often, for 30 to 40 minutes until deeply golden and caramelized. Reduce the heat if they brown too quickly. Set aside.
5. Form and Cook the Burgers:
6. Divide the ground beef into 12 equal portions (2½ to 3 ounces each) and roll into loose balls.
7. Heat a large cast iron skillet or griddle over medium-high heat until very hot. Working in batches, place 2 to 3 beef balls in the skillet. Immediately smash each with a heavy spatula or burger press until very thin. Season with salt and pepper.
8. Cook for 2 to 3 minutes or until the edges are crispy and browned. Flip, top with a slice of cheese, and cook for 1 to 2 minutes, or until the cheese is melted and the patties are cooked through. Transfer to a plate and repeat with remaining patties.
9. Toast the Buns:
10. Lightly butter the brioche buns (optional) and toast cut-side down in the skillet until golden brown.
11. Assemble the Burgers:
12. Spread a generous spoonful of truffle sauce on the bottom bun. Top with a handful of baby arugula, two cheesy burgers, and a spoonful of caramelized onions. Top with the halved bun and secure with a cornichon and toothpick. Repeat with remaining burgers.