Slice the beef as thinly as possible against the grain.
Add the beef to a bowl along with the oyster sauce, shaoxing, 1/2 teaspoon sesame oil, potato starch and black pepper. Stir well to distribute the marinade evenly, separating areas where slices of beef may be sticking together.
Make the stir-fry sauce by stirring together the chicken stock, dark soy sauce, light soy sauce, sugar and potato starch in a small bowl.
Heat a frying pan over high heat until very hot. Add the vegetable oil and swirl to coat the pan. Add the beef and white parts of the scallions and stir-fry until the beef is about half cooked (should still be pink in parts).
Add the stir-fry sauce and then boil until the sauce is thick and coats the beef in a glaze.
Add the green parts of the scallions, reserving a few for garnish, and toss a few times to incorporate. Remove the pan from the heat.