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