MIX meat, 1/2 cup of the cheese, the onion, peppers, olives, mushrooms and seasonings until well blended. Shape into 4 patties.
SPRAY large skillet with cooking spray. Add patties; cover. Cook on medium heat 4 to 6 min. on each side or until cooked through. Top with tomatoes and the remaining 1/2 cup cheese; cover. Cook 1 min. or until cheese is melted.
SERVE on lettuce leaves.