Cut each flank piece of flank steak into 1 ½ inch x 1 inch x ⅛ inch pieces and marinate in soy, shaoxing, sesame oil, sugar, cornstarch, and baking soda.
In a seasoned wok, sear half of the beef off on one side until dark brown, flip and cook for another 10 seconds and set aside. Repeat the process with the rest of the beef.
In the wok saute off half of the broccoli and onions until the veg is tender but still snappy. Repeat with the rest of the broccoli and onions. Add in the garlic and ginger for the last 30 seconds of cooking.
Into the wok add in all of the beef and broccoli and stir fry for about 15-20 seconds until everything is mixed together. Add in stir fry sauce, mix and bring to a boil. Add in cornstarch slurry and toss for another 20 seconds. Remove from the heat and serve with rice.