Mix together the marinade ingredients.
Cut the steak into 3cm wide slices and place them in a bowl. Add the marinade, mix well and leave for 15-20 minutes.
Heat the wok. When piping hot, add 2 tablespoon of oil and wait until the oil is smoking a little - then add the beef and stir-fry for 2 minutes.
Remove the beef, wipe the wok and heat it again. When hot, add the remaining oil, heat to smoking point.
Add the onion, garlic and orange zest and stir-fry for a minute. Add the rice wine, sugar, then stir for 3 minutes.
Put the steak back, add the oyster sauce and warm through.
Serve immediately.