Wash and dry the fish, cut it into slices ¾ in. thick. Put into a fireproof dish with 2 oz. butter, a good pinch of mace and a liberal seasoning of salt and pepper. Cover and cook gently for 1 hr., basting occasionally. When the fish is rather more than half cooked, melt the remaining butter in a saucepan and add the flour. Cook for 2–3 min., put in the milk and stir until boiling. Pour the sauce over the fish and continue to cook gently until done.