Marinate the beef with soy sauce and cornstarch for 15 minutes.
Heat vegetable oil in a large pan or wok over high heat.
Add the marinated beef in a single layer and cook for 2–3 minutes until browned.
Remove beef from the pan and set aside. In the same pan, add garlic and ginger, stir-frying for 30 seconds.
Add the broccoli and a splash of water, covering the pan for 2–3 minutes to steam.
Return the beef to the pan, pour in oyster sauce, and stir everything together.
Cook for an additional 2–3 minutes, allowing the sauce to thicken slightly and coat the ingredients.
Serve hot over rice or noodles, garnished with sesame seeds or chopped scallions if desired.