Cut off tops of green peppers and clean seeds out. Boil in large pot for 5 minutes or until peppers are a dull green. Brown beef with onion. Add uncooked rice and 2 cups of spaghetti sauce. Fill peppers with meat/rice mixture Pour remaining cup of spaghetti sauce over peppers. Cover with aluminum foil and cook for 20 minutes. Remove foil and sprinkle with mozzarella cheese. Cook for another 10 minutes, uncovered.