In a mixing bowl, combine soy sauce, cornstarch, and thinly sliced flank steak. Let it marinate for at least 15 minutes.
In a small bowl, mix oyster sauce, garlic, ginger, and a splash of water to create the stir-fry sauce.
Blanch the broccoli florets in boiling water for 30 seconds, then drain and set aside.
Heat oil in a wok over high heat. Stir-fry the beef for 2-3 minutes until browned but still tender.
Add the broccoli and pour in the prepared stir-fry sauce. Toss everything together, letting the sauce thicken and coat the beef.
Finish with a drizzle of sesame oil and serve over steamed rice or noodles.