Marinate the beef in soy sauce and cornstarch for 10 minutes.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated beef and stir-fry until browned, about 3-4 minutes.
Remove the beef from the skillet and set aside.
In the same skillet, add garlic and ginger, sautéing until fragrant.
Add the broccoli florets and stir-fry for 2-3 minutes until vibrant green.
Return the beef to the skillet with the broccoli.
Pour in the oyster sauce and sesame oil, mixing well to combine.
Stir-fry everything for another 2-3 minutes until heated through.
Serve hot over cooked rice.