First, cut the beef steak into thin slices against the grain. Season the beef with soy sauce, sugar, Shao Xing wine, baking soda and corn starch. Set aside and marinade for 10-15 minutes.
Cut the choy sum into 2" pieces, separate the stem and leaves. Mix all the stir fry sauce ingredients in a bowl, whisk well and set it aside.
In a large wok or pan, drizzle 2 tablespoon of vegetable oil. Heat the wok over medium high heat and add the marinated beef slices. Flatten and sear the beef for 2-3 minutes then flip and cook for another 1-2 minutes. Follow with the garlic, ginger and stir it well for few seconds.
Next add the onion slices, stem part of choy sum and stir fry for 1-2 minutes.
Then add the choy sum leaves, stir for few seconds and pour the sauce in.
Continue cook for another 1-2 minutes until the sauce start to get thick and glossy.
Transfer to serving plate and serve with plain rice or noodles.