In a medium bowl, combine beef, onion, garlic salt and pepper; mix well. Shape into six patties, ¾-inch thick.
In a small bowl, combine the cheese, mushrooms, bacon and mayonnaise; refrigerate.
Grill burgers, covered, over medium heat for 5–7 minutes on each side.
During the last 3 minutes, spoon 1/4 cup of the cheese mixture onto each burger. Serve on buns with lettuce and tomato if desired.