Pour the light and dark soy sauce, chinkiang vinegar, hoisin sauce and sesame oil into a bowl, add the white pepper and mix well.
Slice the steak into slices 2-3mm (⅛" or so) thick across the grain and add it to the sauce.
Cut the tips from the broccoli and then cut the stems into 25-30mm (1-1¼") lengths, plunge them into boiling water and let them sit for 3-5 minutes. Drain, then refresh in cold water.
Top and tail the onion, peel it and cut it into 8 wedges.
Peel the garlic cloves, then slice them as thinly as you can.
Cut the spring onions into 2-3mm thick (just under ⅛") rounds.
Mix the cornflour with the cold water and set aside.
Just before you begin to cook, remove the beef from the marinade and keep the marinade for later.
Heat a wok over a very high heat, and when it is hot, add the cooking oil. Add the onions and stir fry for 60-90 seconds.
Throw in the garlic and stir fry until it becomes fragrant, which will take 15-30 seconds.
Add the beef and stir fry for 30 seconds.
Throw in the broccoli and stir fry for 60 seconds.
Pour in the marinade, the cornflour, water mix, black pepper, and MSG, and stir fry for another 60 seconds.
Add most of the spring onions keeping a few back for garnish, stir to combine then serve.