1Heat oven to 375°F. Coat fish fillets with flour; shake off excess flour. In 12-inch ovenproof skillet, melt butter over medium-low heat. Cook onions in butter about 5 minutes, stirring occasionally, until tender.
2Add fish to skillet. Cook uncovered about 5 minutes or until light brown; carefully turn fish. Pour wine and lemon juice over fish. Sprinkle with salt, pepper and cheese.
3Bake uncovered about 15 minutes or until cheese is melted and fish flakes easily with fork.