In a medium bowl, combine beef, onion, garlic salt, and pepper; mix well.
Shape into patties 3/4 inch thick.
In a small bowl, combine the cheese, mushrooms, bacon, and mayonnaise; refrigerate.
Grill burgers over medium hot coals for 10-12 minutes, turning once. During the last 3 minutes, spoon 1/4 cup of the cheese mixture onto each burger.
Serve on buns with tomato and lettuce if desired.