In large heavy pot, melt butter and sauté onions, green pepper and garlic until soft. Stir in salt, black and cayenne pepper, sugar and tomato paste. Simmer, stirring occasionally, for 20 minutes. Dissolve cornstarch in water; add wine to mixture. Cook for 20 minutes or until sauce thickens. Add crawfish, green onion and Kitchen Bouquet. Refrigerate overnight. Reheat and serve over rice.