Mix together oysters, crumbs, milk, salt, pepper, butter, and optional celery.
Butter a large baking dish and lay one haddock fillet in it.
Brush with egg.
Cover with a generous amount of dressing, and top with second fillet.
Brush top fillet with egg and cover with remaining dressing.
Bake at 375F for 35-40 minutes, or until fish flakes easily with a fork.