Start by mixing together the sauce ingredients in a bowl — oyster sauce, soy sauce, sesame oil, sugar, and beef or chicken stock. Set aside.
Heat 1 – 2 tablespoons of vegetable oil in a wok or large frying pan over medium heat. Add the broccoli and stir-fry for around 4 minutes, or until just tender. Add the ginger and garlic and cook for another 30 seconds until fragrant. Remove everything from the pan, transfer to a plate, and cover to keep warm.
Turn the heat up to high and add another 1 – 2 tablespoons of oil. Add the sliced flank steak in a single layer (cook in batches if needed) and season with salt and pepper. Stir-fry for 3 – 4 minutes per side or until nicely browned and just cooked through.
Return the broccoli, garlic, and ginger to the pan and stir-fry for another 2 minutes until everything is warmed through.
Pour in the sauce and stir to coat. After 30 seconds, add the cornstarch slurry and bring to a boil. Cook for another minute or until the sauce thickens slightly and coats everything nicely.
Serve hot with steamed rice and top with spring onion if you like.
