Mix cornflour, garlic, ginger chilli powder and pepper in a bowl. Add the sliced rump steak and allow to marinade for 15 minutes.
To cook the rice, place the rice and stock into a saucepan, bring to a boil, stir once and put the lid on, reduce the temperature to its lowest setting and cook for 5 minutes. Remove the pan from the heat and allow the rice to steam for 10 minutes. Do not remove the lid and do not stir the rice.
To cook the beef, heat the oil in a wok over high heat and stir-fry the onion for 2 minutes until it just begins to soften, add the marinated steak and stir fry for 2 minutes until almost cooked.
Add the pak choi to the pan and continue to cook for a minute or two until tender. Stir in soy sauce, beef stock and oyster sauce and cook for 1 minute or until the sauce thickens.
Spoon rice into warm serving bowls and top with black pepper beef.