Add the thinly sliced beef to a nonstick skillet or wok. Sear for 2–3 minutes per side until browned. Remove and set aside. (I seasoned the beef with garlic powder, onion powder, salt, and pepper.)
Whisk the soy sauce, oyster sauce, vinegar, garlic, ginger, and sesame oil.
Add the broccoli and a splash of water to the skillet. Cover and steam 2-3 minutes until bright green and tender-crisp.
Return the beef to the pan and pour the sauce over everything.
Garnish with green onions and sesame seeds (optional).