Trim the fat off the steak, then slice it thinly. Pop it into a mixing bowl and add the cornflour, then stir to coat it.
Heat some oil in a large frying pan or a wok over a high heat. Stir fry the beef until it’s browned all over.
Chuck in the garlic, ginger, and sliced onion and stir fry for 30 seconds.
Add in the pak choi and cook for another minute before pouring in the oyster sauce, soy sauce, mirin, sesame oil, soft brown sugar and a splash of water.
Bring it to the boil and cook until it’s sticky and glossy.
Serve with rice and enjoy.