New England Fish Chowder

2 tablespoons margarine
3 tablespoons shredded carrot
2 tablespoons diced celery
2 tablespoons minced fresh onion
2 tablespoons plus 1 teaspoon all-purpose flour
3 1/2 cups skim milk, divided
2 cups diced peeled baking potato
1/2 teaspoon salt
1/4 teaspoon pepper
1 pound cod or other lean white fish fillets, cut into 1-inch pieces

Melt margarine in a saucepan over medium heat. Add carrot, celery, and onion; saut? 2 minutes.
Stir in flour; gradually add 2 1/2 cups milk, stirring constantly with a whisk.
Add potato, salt, and pepper; bring to a boil. Reduce heat. Simmer, uncovered, 30 minutes; stir occasionally.
Add fish and 1 cup milk; cook an additional 10 minutes or until fish is done.
Serve with oyster crackers, if desired.