Put the beef in a large bowl, season with pepper, soy sauce, sherry, ginger and half the oil and marinate for 20 minutes. Meanwhile, heat the remaining oil in a large non-stick wok or pan. Remove the beef from the marinade (reserve this for later) and stir fry the meat for 4-5 minutes.
Add the spring onions, pak choi or broccoli, bean sprouts, Thai fish sauce, chilli and noodles. Stir fry for 1-2 minutes, tossing occasionally.
Add the reserved marinade and cook for a further minute. Add the lemon zest and serve immediately.