In a saucepan, combine the ground pork, water, nutmeg, allspice, salt, pepper and onion. Cook over moderate heat for 30 minutes, stirring often to prevent sticking. Remove from heat; add the mashed potatoes; and mix well. Cool. Fill two pastry-lined plates;adjust top crusts, slashing pastry to allow for the steam to escape as the pies bake. Bake in a hot oven, 425 degrees F., For 40 minutes. Serve hot or cold.