Whisk together the oyster sauce, sesame oil, sherry, soy sauce, sugar, and cornstarch in a bowl, and stir until the sugar has dissolved. Place the steak pieces into a shallow bowl, pour the oyster sauce mixture over the meat, stir to coat well, and marinate for at least 30 minutes in refrigerator.
Mix broccoli, ginger, and garlic powder in large bowl.
Sauté broccoli in hot oil until bright green and almost tender, 5 to 7 minutes. Remove the broccoli from the wok, and set aside.
Sauté the beef and marinade until the sauce forms a glaze, and the meat is just barely pink. Return the cooked broccoli to the wok, and stir until the meat and broccoli are heated through.