Put enough shortening in a large pot to cover the bottom. Get hot then add celery, shallots, onions, bell peppers, onions, parsley and lemons. Cook slowly covered until soft and onions transparent. Remove from pot and set aside. Heat margarine in pot; add flour slowly, stirring to brown the roux. Now add the cooked vegetable mix. Cook low for 30 min, stirring occasionally. Add mushrooms, tomatoes, tap water, bay leaves and Worcestershire. Cook on low for 1-1/2 hours, stirring occasionally. Add fish fillets, whole or pieces, and cook for another hour over low. Add oysters 20 minutes before serving over hot rice. Also for: Any fish; you name it.