Preheat oven to 400 degrees. Trim mushroom; cut into thick slices; set aside. In a small cup, combine olive oil, rosemary, salt and pepper, stir until well blended. In a 13 x 9 x 2-inch oven-proof dish, combine mushrooms, potatoes, onion and garlic; add 3 tablespoons of the seasoned oil; toss. Bake for 10 minutes; stir well. Continue to bake until vegetables are tender, about 15 minutes longer. Cut fish into 4 portions. Place fish on top of potatoes; drizzle remaining 1 tablespoon seasoned oil over fish. Bake just until fish is opaque in the thickest part, 10 to 12 minutes.
Per Serving: 408 Calories (32.8% from fat); 15g Fat (2g Sat, 10g Mono, 2g Poly); 31g Protein; 38g Carbohydrate; 4g Dietary Fiber; 61mg Cholesterol; 225mg Sodium. Food Exchanges: 3 1/2 Lean Meat; 1 1/2 Vegetable; 2 1/2 Fat.