Finely slice the pork and place in a mixing bowl with the rice wine or sherry, soy sauce, sesame oil and cornstarch. Stir to combine, then set aside to marinate for 15 minutes.
Meanwhile, trim the base of the bok choy and wash the stalks thoroughly. Cut off the leaves and set aside, then chop the stalk into slices about 1in wide and set aside separately from the leaves. Finely shred the root ginger to give about 1 tbsp of strands.
Heat the vegetable oil in a wok. When very hot and slightly smoking, add the pork mixture and stir-fry for 2-3 minutes, or until brown. Remove the pork from the wok using a slotted spoon and set the meat aside to drain.
Add the ginger and garlic to the wok and stir-fry for 1-2 minutes or until golden. Add the chopped stalks of the bok choy and stir-fry for 2 minutes, then add the leaves, oyster sauce and 3-4 tbsp water and continue stir-frying until the sauce is bubbling.
Return the pork to the wok and heat through with the bok choy for 1-2 minutes, then serve.