Place The Fillets In A Shallow 2-Qt. Baking Dish Coated With Nonstick Cooking Spray. Sprinkle With Lemon Juice And Pepper. Bake, Uncovered, At 350 Degrees F For 20-25 Minutes Or Until Fish Flakes Easily With A Fork. Meanwhile, In A Small Skillet, Saute The Mushrooms And Garlic In Butter Until Tender. Add Shrimp; Cook For 2-4 Minutes Or Until Shrimp Turn Pink. Spoon Over Fish. Sprinkle With Onion.