Cut 1/4-inch slice from top of each green pepper; remove seeds. Cook green peppers about 5 minutes in enough boiling salted water to cover them; drain. Pan-fry ground beef, onion and celery until lightly browned. Add remaining ingredients; combine thoroughly. Fill green pepeprs with meat mixture. Stand upright in shallow baking pan; add small amount of water. Bake in preheated moderate oven (350 F.) about 30 minutes. Serve hot with mushroom sauce made by heating together mushroom soup and milk.