Cutting against the grain, slice the flank steak into thin strips. Combine the steak with the marinade ingredients. Mix well. Combine the ingredients for the stir-fry sauce. Heat the oil in a wok or frying pan over high heat. Add the beef and cook, stir-frying, for about 2 minutes or until just cooked. Transfer the beef to a large tray or plate. Return the same pan to the heat (no need to clean it). Add in the onion (add a little more oil if there isn’t any left in the pan), garlic and chillies. Stir-fry for about a minute. Then add the stir-fry sauce. Allow to simmer for half a minute. Then add the corn flour mixture and simmer until thickened. Add the beef back into the sauce along with the spring onion and toss until well combined. Serve warm.