For the beef marinade, combine beef slices with brown sugar, cornstarch, soy sauce, and rice wine in a bowl. Toss well and let it sit for 10 minutes.
For the sauce, mix all the sauce ingredient in another bowl; set aside.
To stir-fry, heat 1 tablespoon of oil in a wok or skillet over high heat until very hot. Add the beef and let it sear for few seconds, then stir-fry until they are just browned. Transfer to a plate and set aside.
Heat the remaining oil in a wok until hot, add broccoli and onion, and stir-fry for 1 minute. Make sure to maintain the high heat. Add 2 tablespoon of water to create the steam so that can cook the broccoli florets to a tender crisp.; about 2 minutes. Add more water if needed.
When the broccoli florets are tender yet crisp, return the beef and pour the sauce over. Continue to stir-fry until the sauce gets thickened, about 30-60 seconds. Serve warm with rice or noodles