Melt margarine and cook onion and celery until limp. Add 3 cups water. Cover and cook about 2 hours on low heat. Uncover, add fat and cornstarch to thicken. Cook and stir as bottom will stick, about 1/2 hour longer. Add crawfish tails and cook about 20 minutes longer. Season to taste Serve over hot rice.
Cook early so flavor can become enhanced.
Cajun Country -- New Iberia, Louisiana