In a nonstick skillet, cook onion in oil over medium heat for about 2 minutes. Stir in zucchini and mushrooms; cook and stir 2 minutes longer. Sprinkle with pepper and garlic powder; stir in lemon juice. Cook and stir 30 seconds longer. Remove from heat; stir in 1 teaspoon cheese.
Place fillets in a 13-in. x 9-in. baking dish coated with cooking spray. Top each fillet with about 1/4 cup onion mixture. Drizzle with butter and sprinkle with remaining Parmesan cheese.
Bake, uncovered, at 375° for 18-22 minutes or until fish flakes easily with a fork.