Remove the skin, which would impart a disagreeable oily taste to the dish, and cut the fish into pieces 1½ in. long. Bring the stock or water to simmering point, put in the fish, onion (sliced), herbs, mace, cloves, a little salt and pepper and simmer gently for 20 min. Meanwhile melt the butter in a saucepan, add the flour, stir and cook slowly for 3 min., without browning. Strain on the liquor from the fish, stir until boiling, boil for 5 min., then season to taste. Strain over the fish and serve.