Marinade
On a cutting board, slice the pork tenderloin into thin slices or 3/4-inch cubes. Combine marinade ingredients in a medium-sized bowl and mix well. Add the slices of pork tenderloin to the bowl and let stand while preparing the other ingredients. Meanwhile, wash the watercress and pat dry. Cut the roots off of the watercress, leaving about 1 inch. Roughly chop the watercress into 1-inch pieces and set aside. Finely chop the green onions, using both the white and green parts. In a large saucepan or wok, bring the chicken stock and water to a boil. Stir in the sugar and let simmer for a minute, add the marinated pork. Bring back to a boil and cook for about 3 minutes. Add the watercress and the chopped green onions. Boil for 2 – 3 minutes, until the pork is cooked and the watercress leaves are a bright green and wilted. Ladle into soup bowl while hot. Makes 6 to 8 servings.