Melt margarine in a small aucepan over low heat. Add onions and celery; saute until tender. Stir in flour, salt and pepper; cook 1 minute, stirring constatntly.
Gradually add milk; stir well. Cook over medium heat until thickened and bubbly, stirring constantly. Add shredded swiss cheese;reduce heat and cook until cheese mells, stirring constantly. Set aside. Arrange fillets in a single layer in a 10 x 6 x 2 inch baking dish coated with cooking spray. Pour cheese mixture evenly over fillets. Cover and bake at 350°F for 25 minutes, or until fillets flake easily when tested with a fork. Sprinkle with parsley. Yield 4 servings.
