Meatballs: Finely chop dozen of the water chestnuts. Reserve the remaining ones for garnish. Combine the pork, ginger, chopped water chestnuts, soy sauce, cornstarch, salt and pepper. Mix well and form into balls 2 cm in diameter. These may be made in advance and frozen. Be sure to thaw completely in refrigerator before poaching. Soup: Bring the vegetable stock and chicken stock to a simmer in a large pot. Put a fourth of the meatballs in the broth and poach until they rise to the top. Remove and keep warm. Repeat with the remaining meatballs. About 20 minutes prior to serving, return stock to a boil and add the meatballs. Cook 10-15 minutes (Note: Meatballs will be slightly pink in the center even after the second cooking.) Season with salt and black pepper to taste. Turn heat to medium low. Add the watercress and green onions. Cook, uncovered, for a few minutes until watercress is slightly wilted and bright green in color. Add the remaining water chestnuts and cook for 1 minute so that they remain crisp. Serve immediately. Serve as the first course of a traditional Chinese dinner or alone as a light nutritious supper.