Cut tops off peppers and remove seeds.
Parboil in salted water for 5 minutes; drain.
Stuff peppers with half of macaroni.
Beat egg; add sour cream, mustard, salt, pepper, onion and celery and pour over half of macaroni.
Sprinkle with half of shredded cheese.
Add remaining macaroni to peppers, pour in remaining sour cream mixture and top with Cheddar cheese.
Bake at 350° for 30 minutes.