In small bowl, combine cheese, 3 tablespoons mustard, onion, and horseradish; set aside. In medium bowl, combine beef and remaining mustard; shape mixture into 4 patties. Grill burgers over medium heat for 5 minutes each side or until desired doneness; top with cheese mixture and cook until cheese melts, about 2 minutes. Top each roll bottom with 1/4 cup lettuce, 1 tomato slice, burger, 2 bacon pieces, and roll top. Yield: 4 servings.