Baked Fish with Onions

1 1/2-2 lbs fish fillets, such as striped bass, turbot, grouper, skin on kosher salt , to taste 1/2 cup fresh lemon juice 5 tablespoons extra virgin olive oil[...]

2 tablespoons olive oil 5 tablespoons butter 6 cups sliced onions 5 tablespoons flour 2 1/2 cups milk salt and pepper 1 cup grated swiss cheese or 1 cup[...]