Cook peppers in boiling salted water for 5 minutes; drain well (turn peppers upside down to drain). Saute onion in butter until tender but not brown; add meat and saute until brown; combine with remaining ingredients except 1/4C. cheese.
Stuff peppers with meat mixture. Place in a deep baking dish; pour 1/2-inch water in bottom of baking dish. Bake at 350 degrees F. for 15 minutes; sprinkle tops of filled peppers with remaining cheese.
Continue to bake 15 minutes or until cheese is melted and meat is heated through.