4 (1/2-inch thick) slices red onion, not separated into rings
2 teaspoons olive oil
4 ounces sharp Cheddar cheese, sliced
4 large slices sourdough bread, cut in half
01Heat grill to high.
02In a medium bowl, gently combine beef, horseradish, mustard, salt and pepper. Shape into four 3/4-inch-thick burgers.
03 Brush onion slices lightly with olive oil.
04Brush grill with wad of paper towel dipped in oil to prevent sticking. Place burgers and onion slices on grill. Cook burgers to desired degree of doneness, about approximately 4 minutes per side for medium rare. Cook onions until browned, approximately 4 minutes per side. Toast sourdough bread lightly.
05Top burgers with cheese and cover with grill lid (or foil)until cheese is melted, approximately 1 minute. Serve on lettuce-topped bread, garnished with grilled onions separated into rings.