Sprinkle Spinach In A 13-In. X 9-In. X 2-In. Baking Dish Coated With Nonstick Cooking Spray. Top With Fillets. In A Saucepan, Saute Onion In Butter Until Tender. Stir In Flour, Salt, Pepper And Nutmeg Until Blended. Gradually Add Milk. Bring To A Boil; Cook And Stir For 2 Minutes Or Until Thickened And Bubbly. Pour Over Fillets; Sprinkle With Parmesan Cheese And Paprika. Bake, Uncovered, At 350 Degrees F For 20 Minutes Or Until Fish Flakes Easily With A Fork.