Put enough water in the pot for the fish to swim in easily. Add half a cupful of vinegar, a teaspoonful of salt, an onion, a dozen black peppers, and a blade of mace. Sew up the fish in a piece of clean mosquito netting, fitted to its shape. Heat slowly for the first half hour, then boil twelve minutes to the pound, quite fast. Unwrap, and pour over it a cup of drawn butter, based upon the liquor in which the fish was boiled, with the juice of half a lemon stirred into it. Garnish with sliced lemon.