Preheat oven to 350°.
Melt margarine in a large nonstick skillet over medium-high heat until hot. Add onion, bell pepper, celery, and garlic; sauté 5 minutes. Stir in flour; cook 2 minutes. Stir in milk and tomatoes; cook 7 minutes or until thick. Stir in pasta and remaining ingredients. Spoon fettuccine mixture into a 13 x 9-inch baking dish; bake at 350° for 25 minutes.
Note: Packages of cooked peeled crayfish can be found in the frozen-food setion of many supermarkets.