Blondie's Delicious Burgers

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.