Ingredients
1/3 cup mayonnaise
3 tablespoons Dijon mustard
2 tablespoons freshly grated horseradish
Kosher salt and freshly ground pepper
1 1/2 pounds ground beef (85 percent lean)
4 brioche buns, split
Vegetable oil, for drizzling and grill
1 large sweet onion, such as Vidalia (12 ounces), sliced into 1/2-inch-thick rounds
2 tablespoons Worcestershire sauce
4 slices sharp white cheddar
2 beefsteak tomatoes, sliced into thick rounds
Flaky sea salt, such as Jacobsen
Mixed greens, such as watercress and chopped radicchio, and cornichons, for serving
Preparation
Stir together mayonnaise, 1 tablespoon mustard, and horseradish. Season lightly with salt and pepper. Form beef into 4 patties, each slightly larger in circumference than buns. Make a small indent in center of each patty with your thumb (to keep them from puffing up on the grill). Cover loosely and refrigerate until ready to cook.
Heat grill to medium-high, and place a large straight-sided skillet with a lid nearby. Brush grates with oil. Drizzle onions lightly with oil; season with salt and pepper. Grill, flipping once, until well charred and crisp-tender (they should still maintain a bit of crunch), 7 to 9 minutes. Transfer to skillet, drizzle with Worcestershire sauce, and cover with lid. Cover grill and return to medium-high heat.
Generously season indented sides of patties with salt and pepper. Brush with remaining 2 tablespoons mustard. Brush grates with oil. Place patties on grill, indented-sides down, and generously season other sides. Cook until lightly charred, 3 to 4 minutes. Flip patties, top each with cheese, and cover grill. Continue to cook, moving occasionally as needed to avoid flare-ups, until cheese has melted and burgers are cooked through, 3 to 4 minutes more. Transfer to skillet; cover with lid. Grill buns, cut-sides down, until lightly toasted, about 1 minute.
Sprinkle tomatoes with flaky salt. Spread both sides of buns with horseradish mixture, then add patties, onions, and greens. Serve, with salted tomatoes and cornichons alongside.
