In a bowl, toss the beef with soy sauce, oyster sauce, cornstarch, and a pinch of salt. Let it marinate for 15 minutes.
Heat oil in a large skillet or wok over medium-high heat.
Sear the marinated beef until browned and mostly cooked through. Remove from skillet and set aside.
In the same skillet, add a bit more oil if needed and sauté garlic, onion, and bell peppers for 3–4 minutes until crisp-tender.
Return beef to skillet, add beef broth, and ground black pepper. Stir everything together and cook for 2–3 minutes until sauce thickens.
Serve hot over cooked white rice.