Ingredients
- ⅓ cup mayonnaise
- 3 tablespoons Dijon mustard
- 2 tablespoons freshly grated horseradish
- Kosher salt and freshly ground pepper
- 1 ½ 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 ½-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
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