Place ribs on rack in shallow pan. Do not cover. Do not add water. On each piece place one lemon slice. Sprinkle the onion over the ribs. Bake in hot oven (450º) for about 30 minutes. In a saucepan, mix the ingredients for the sauce. Simmer for 15 minutes or until slightly thickened. Pour sauce over the ribs and continue baking in moderate oven (350º) for 1½ – 2 hours. Baste with sauce every 15 minutes. If sauce thickens add a little hot water. To prevent ribs from browning too much, over the last 30 minutes of baking.