Make a light roux with the margarine and flour. Add the tomatoes and let simmer about 3 minutes. Add onions, bell pepper and celery; saute until clear. Season fish with salt and pepper. Add the fish to the pot and let simmer on lower heat for about 20 minutes stirring very gently. Add onion tops and parsley and let cook 5 minutes longer. Season to taste. Serve over hot rice.
Cajun Country -- New Iberia, Louisiana