Cut the beef into small thin slices and rub these with the salt and 1/2 tablespoon of oil. Mix the soy sauce, cornstarch, water, sherry and stock in a bowl until well blended. Finely chop the ginger.
Heat the oil in a frying pan. When hot add the beef, spreading the pieces out, and stir-fry them over a high heat for 1 minute. Add the ginger, followed by the oyster sauce and sugar, and continue to stir-fry for 30 seconds.
Pour in the sauce mixture from the bowl, and continue to stir-fry over a high heat for a further minute. Serve immediately on a well-heated dish.