Cook celery and onion in the water for 5 minutes; do not drain. Mix with lamb, 1 cup of the gravy, parsley, and peas. Put in shallow 1-1/2 quart baking dish. Add salt and pepper to taste. Spread with potato and dot with butter. Sprinkle with paprika. Bake in preheated hot oven (400 degrees) for about 30 minutes. Serve with remaining gravy, heated.