Baked Fish au Gratin

Heat oven to 400 degrees. Place half fish in greased baking dish. Cover with 4 slices cheese. Top with another layer of fish and cheese. Sprinkle with thyme and parsley. Saute onions in oil over medium heat. Remove onions from heat and add flour, salt and pepper. Return to heat and add milk. Stir over medium heat until thick. Pour over fish. Bake 20-30 minutes.