Place pork, green onions and water chestnuts in a 6-qt electric pressure cooker. Mix hoisin sauce, water, soy sauce, sherry, garlic, gingerroot, corn syrup, marmalade, pie spice and pepper flakes in a bowl. Pour over pork. Lock lid and close pressure-release valve. Adjust to pressure-cook on high for 25 minutes. Allow pressure to naturally release for 10 minutes and then quick-release any remaining pressure. , Remove pork to a serving platter; keep warm. Skim fat from cooking juices. Select saute setting and adjust for medium heat. Bring to a boil. In a small bowl, mix cornstarch and water until smooth. Gradually stir cornstarch mixture into the pressure cooker. Bring to a boil; cook and stir until thickened, about 2 minutes. , Serve ribs with sauce and, if desired, rice and additional green onions.