In a bowl, combine beef and garlic salt. Shape into eight patties; top four of the patties with cheese. Cover with remaining patties; press edges to seal. Refrigerate. In a saucepan, combine the pizza sauce, mushrooms and oregano; cover and simmer for 10 minutes, stirring occasionally. In a skillet, saute onion in butter until tender; set aside. Pan-fry, grill or broil burgers until no longer pink. Spread bottoms of buns with a little of the sauce; top with burgers, onion and remaining sauce. Replace tops.