Preheat the oven to 350 degrees F. Saute onions in the oil in a skillet until translucent. Stir in the flour and salt. Add the milk. Cook until thickened, stirring constantly. Add the carrots, cheese and lemon juice. Place the fish in a 7X11 inch baking dish. Pour the sauce over the fish. Bake for 25 minutes or until 145 degrees on a meat thermometer. Serve with hot cooked rice or pasta.
Notes
You may substitute broth for the milk. Also you may add other vegetables, such as peppers, peas, etc.