These richly flavored dumplings are served with a soy and sesame oil dipping sauce.
Fill a large saucepan with cold water and bring to a boil. Add the heads of bok choy and gently boil until tender, about 2 to 3 minutes. Using a slotted spoon, remove bok choy from saucepan and plunge into a bowl of ice water. Drain, discarding the liquid, and finely chop. Place bok choy in a clean kitchen towel or a double thickness of cheesecloth and squeeze out all of the water. Transfer to a bowl and set aside.
In a medium bowl, combine pork, egg, cornstarch, oil, 1 tablespoon soy sauce, ginger, fish sauce, salt, and pepper. Add chopped bok choy and mix well.
Place a dumpling wrapper in one hand and spoon about 1 tablespoon of meat mixture into the middle of the wrapper. Dampen fingertips and moisten entire edge of the wrapper. Fold dumpling in half to enclose filling and pinch the two edges together tightly, forming a semicircle. Repeat this process until all the filling is used up.
Bring a large pot of water to a boil. Add as many dumplings as will fit without crowding. Gently stir them up from the bottom to prevent sticking, cover, and bring back to a boil, about 1 minute. Uncover, add 1/4 cup cold water, replace cover, and bring water back to a boil. Boil for about 2 more minutes. Remove with a slotted spoon and keep warm. Continue until all have been cooked.
Make the dipping sauce by combining the remaining soy sauce, sugar, sesame oil, and scallion. Mix well, transfer to small serving dishes, and serve with the dumplings.
Dumpling wrappers can be found in the produce section of many supermarkets.