Slice pork against the grain into thin strips about 5 cm by 1 cm. Seperate outer fleshy stalks from center flower stalk of bok choy. Use center stalk in another stir-fry dish or in soup. Wash outer stalks, and diagonally slice into 8 cm pieces. The meatier ends of the stalks should be sliced a little thinner. Swirl oil around in very hot wok. When oil begins to smoke, add pork and stir-fry until meat is seared (about 1 minute). Add bok choy, garlic and ginger. Stir-fry until green leaves are bright and shrivelled, stirring and tossing constantly so stalks won't burn. Add salt, stock, sugar and dry sherry. When liquid begins to boil, cover wok and steam for 30 seconds or less on high heat, until stalks are tender but still crisp. Push ingredients up side of wok; dribble cornstarch paste into liquid and stir to make a light sauce. Recombine and add MSG. Serve immediately.