Wash the fish in vinegar and water and cut it into thick slices. Slice the onions. Melt oz. of the butter in a saucepan, fry the onion until brown, then add the stock, mushrooms, herbs, nutmeg and seasoning. When warm, add the fish and simmer gently for 30–40 min. Meanwhile knead together the flour and remaining butter. Take out the fish and keep it hot. Add the butter and flour to the contents of the saucepan, simmer and stir until the sauce is cooked smoothly. Place the fish on a hot dish, strain the sauce over and garnish with the mushrooms and sippets of toasted bread.