In a bowl, combine pork, watercress, ginger, sesame oil, soy sauce, and egg whites. Refrigerate for 30 minutes. Place 1 teaspoon mixture in the center of a wrapper. Fold 1/3 of wrapper over filling on each side; bring open ends together and seal by fluting edges. (The potsticker should look like a purse.) Repeat until all wrappers or filling is used. In a large skillet, heat 2 tablespoons oil. Arrange potstickers in rows in the heated skillet. Add 1/2 cup water, cover skillet, lower heat and cook for 7 to 8 minutes or until water is evaporated and bottoms of potstickers are golden brown. Repeat until all potstickers are cooked. Combine the remaining ingredients for dipping sauce. Makes 3 dozen potstickers.