Make the sauce: In a bowl, whisk together soy sauce, oyster sauce, cornstarch, sugar, garlic, broth, sesame oil, and pepper.
Sear the beef: Heat oil in a large skillet or wok over high heat. Stir-fry beef until browned, 2–3 minutes. Remove and set aside. Cook veggies: Add peppers and onions; stir-fry until just tender, 3–4 minutes.Combine: Return beef to the pan. Pour in the sauce and stir until everything is coated and sauce thickens, 1–2 minutes.
Serve hot over rice or noodles.
