In a bowl, combine Five Spice Powder, sherry, soy sauce, garlic and ginger. Add pork, stir well and let stand 30 minutes. Cut onions in eighths and separate in layers. Heat 2 tablespoons of oil in a skillet or wok. Drain pork, reserve marinade. Add pork to oil and stir-fry 5 minutes. Remove from skillet and keep warm. Add remaining oil to skillet. Add onions, bell peppers, mushrooms and water chestnuts. Stir-fry 3 minutes. Add vegetable mixture to pork. Blend cornstarch with reserved marinade and 2 tablespoons of stock. Add remaining stock to skillet and bring to a boil. Add cornstarch mixture and cook 2 minutes, stirring constantly. Add pork and vegetables to stock and heat through, stirring constantly. Garnish with leek and green onion curls, if desired, and serve hot.