Two or 3 pound of fillet fish; 1 dessert spoon of butter; 1 cup of water, 1 dessert spoon of flour; 1 onion; 1 dessert spoon of vinegar; salt and chopped parsley.
Cut the fish into small pieces (about 2.5 inches). Slice the onion thinly and fry in the butter until a golden brown, add flour and cook a few minutes before adding vinegar and water, then boil altogether for a few minutes and add pepper and salt. Arrange the pieces of fish in the casserole, pour over the sauce and cover. Cook in a moderate oven about 15 minutes or until cooked. Serve in casserole, sprinkled with parsley and garnish with lemon.