Wash the mustard greens, and discard the tough stems.
Bring water to a boil in a large pot, add the greens, and cook for 2 minutes.
Drain, rinse with cold water to stop the cooking, drain again, and cut into long, diagonal strips.
In a bowl, mix the cornstarch with water, then stir in the sugar, soy sauce and rice wine. Slice the pork thinly, and add it to the mixture; marinate for about 20 minutes, stirring occasionally.
Heat the oil in a wok or large pan, add the black bean sauce, and stir-fry for 1 minute.
Add the pork, and stir-fry for 2 minutes.
Add the mustard greens and water, mix well, and simmer gently for 4–5 minutes, until the pork is cooked through and the greens are tender.