In a large bowl, combine beef and garlic salt. Shape into eight patties; top four of patties with cheese. Cover with remaining patties; press edges to seal. Refrigerate.
Meanwhile, in a large saucepan, combine the pizza sauce, mushrooms and oregano; cover and simmer for 10 minutes, stirring occasionally. In a small 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.