Cook in butter half a dozen small onions cut in two; add a tablespoonful of flour. Boil, in another saucepan, two parts of water to one part of red wine. Pour the boiling mixture on the onions, add salt and pepper, half a dozen prunes, cut in two and stoned. Let this simmer about one hour and a half. Add your fish, cut in slices, about twenty minutes before serving. Surround the dish with croûtons.