- 1/4 cup Dijon mustard
- 1 tablespoon prepared horseradish, drained
- 1 large Vidalia onion, sliced crosswise into 1/2-inch-thick slices
- 2 tablespoons canola oil
- Kosher salt and freshly ground black pepper
- 2 pounds ground chuck
- 8 slices Cheddar cheese (each 1/4 inch thick), preferably a mix of white and yellow
- 4 sesame seed hamburger buns
- 4 slices beefsteak tomato
- 4 lettuce leaves
Whisk together the mustard and horseradish in a small bowl; set aside.
Preheat the grill to high or a grill pan over high heat.
Brush the onion slices with the oil on both sides and season with salt and pepper. Grill the onion slices for 3 to 4 minutes on each side, until lightly golden brown.
While the onion is grilling, form the meat into 4 burgers. Season the burgers on both sides with salt and pepper. Grill for 3 to 4 minutes on each side for medium. Add 2 slices of the cheese to the top of each burger, cover the grill, and let melt, about 1 minute.
Place the burgers on the buns sandwiched with onion, tomato, lettuce, and a dollop of horseradish mustard.