Place the sliced rump steak into a bowl with the cornflour, soy sauce, shaoxing wine, sesame oil and bicarbonate of soda. Mix until the beef is evenly coated, then leave to marinate for while preparing the remaining ingredients.
Combine the oyster sauce, light soy sauce, dark soy sauce, brown sugar, chicken stock powder and water in a jug. Whisk until well combined.
Bring a saucepan of water to the boil. Add the broccoli florets and cook for 1 minute until bright green and slightly tender. Drain well.
Heat 1 tablespoon of vegetable oil in a large wok or frying pan over high heat. Add the beef in a single layer and cook for 1 minute without stirring. Toss and cook for another minute until browned but not fully cooked through. Remove from the pan.
Add the remaining oil to the wok. Cook the onion for 2 minutes until softened slightly. Add the garlic and ginger and stir fry for 30 seconds until fragrant.
Return the beef to the wok along with the broccoli. Give the sauce a quick stir to loosen the cornflour, then pour it into the wok and toss everything together for 1 to 2 minutes until thickened and glossy, adding a splash of extra water to loosen the sauce if required.
Serve hot over jasmine rice with sliced spring onions and sesame seeds.