Slice steak across the grain on the diagonal to form strips no more than half a centimetre (3/16″) thick.
In a large bowl, whisk together the ingredients for the marinade. Add the steak and mix until well combined.
In a separate bowl, mix together the ingredients for the stir-fry sauce.
Heat the oil in a wok or large frying pan over high heat. Add the beef and spread it out in the pan so there’s maximum surface area coming into contact with the hot pan. Allow to sear for 1 minute, then toss and stir-fry. Repeat the spreading, searing and tossing business for 1–2 minutes or until the beef is starting to crisp and brown on the edges. Transfer 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), ginger, garlic and chillies. Stir-fry for about a minute, then add the stir-fry sauce. Allow to simmer for half a minute. Next, add the cornflour mixture and simmer until thickened. Add the beef back into the sauce along with the spring onion and toss until well combined. Serve warm with steamed rice.
