If frozen, make sure fish is defrosted. Place in shallow baking pan, lined with aluminum foil, and lightly greased with margarine.
In a small cup, mix together the lemon juice, oil, paprika, and ½ teaspoon of salt. Brush the tops and sides of the fish fillets with mixture. Bake at 350 degrees Fahrenheit for 30 to40 minutes, or until the fish flakes when tested with a fork.
While fish is baking, cook the flour, margarine, dry milk, and remaining ½ teaspoon of salt in a small saucepan over low heat, stirring until smooth. Add two cups of water to the flour mixture and cook over low heat, stirring until smooth and thickened.
Add chopped parsley and mustard to sauce and stir well to mix. Season with white pepper to taste. Serve 1/3 cup of the sauce over each fillet.