Discard any fat on the beef and cut it into thin strips. Mix together the soy sauce, vinegar, ginger, garlic, chillies, spring onions and sugar and marinate the beef in it for 30 minutes.
Heat a wok or large frying pan and when it is very hot, pour in half the oil, swirling it around to coat the sides. Stir-fry the greens for 3 minutes and remove them from the wok. Drain the beef