Use shrimp if you can't get crawfish.
Melt margarine in dutch oven (not iron pot). Season crawfish tails generously with Creole seasoning. Add paprika to margarine. Saute crawfish about 5 min. Remove crawfish and set aside. To pot add onions, bell pepper and garlic. Saute well at least 10 minutes. Return crawfish tails to pot and add 2 cups water and Worcestershire sauce. Stir and simmer slowly about 40 minutes. Check for taste, add more seasoning if necessary. add mixture of cornstarch and water slowly until slightly thickened.
Serve with rice and garnish with onion tops and parsley.