Smashburgers with Special Truffle Sauce

1. In a small bowl, combine the mayo, chopped cornichons, Dijon, lemon juice, and truffle oil. Season with salt & pepper to taste. Stir well, then cover and refrigerate until ready to use. This keeps well for 5 days.
2. Thinly slice 2 large yellow onions. In a large skillet over medium heat, warm 1–2 tablespoons of oil. Add onions and a pinch of salt. Cook, stirring often, until deeply golden and caramelized, about 30–40 minutes. Set aside.
3. Divide the ground beef into 12 equal portions (about 2.5 to 3 ounces each) and roll into loose balls. Heat a large cast iron skillet or griddle over medium-high heat until very hot. Working in batches, place 2–3 beef balls in the skillet. Immediately smash each with a heavy spatula or burger press until very thin. Season with salt & pepper.
4. Cook for about 2–3 minutes, or until the edges are crispy and browned. Flip, top with a slice of cheese, and cook for 1–2 more minutes, or until the cheese is melted and the patties are cooked through. Transfer to a plate and repeat with remaining patties.
5. Lightly butter the brioche buns and toast cut-side down in the skillet until golden brown.
6. Assemble and enjoy!