Prepare the beef by thinly slicing the flank steak against the grain.
Marinate the beef in a mixture of soy sauce, cornstarch, and half the garlic and ginger.
While the beef marinates, prepare the broccoli by washing and cutting it into bite-sized florets.
Heat vegetable oil in a large wok or skillet over high heat until it shimmers.
Add the marinated beef to the hot pan in a single layer and sear for 1-2 minutes without stirring.
Stir-fry the beef until browned and cooked through, then remove from the pan and set aside.
In the same pan, add a little more oil if needed and toss in the remaining garlic, ginger, and broccoli florets.
Stir-fry the broccoli for 2-3 minutes until bright green and slightly tender yet crisp.
Return the beef to the pan, add in the oyster sauce, sesame oil, and a splash of water or broth, if desired.
Toss everything together for an additional minute to ensure the beef and broccoli are coated in sauce.