Slice the beef thinly and marinate it with soy sauce and cornstarch for 15–20 minutes.
Heat oil in a wok over high heat and stir-fry the beef until lightly browned. Remove and set aside.
In the same wok, sauté minced garlic and fresh ginger until fragrant.
Add sliced onions and bell peppers, stir-frying until slightly tender but still crisp.
Stir in soy sauce, oyster sauce, freshly cracked black pepper, and a splash of water to create the sauce.
Return the beef to the wok and toss until evenly coated and heated through.
Serve hot with steamed rice or noodles.