Slice beef thinly against the grain
Marinate with soy sauce, cornstarch, black pepper, and a little oil
Heat oil in a pan or wok over high heat
Stir-fry the beef until browned, then remove
Add sliced onions, bell peppers, and garlic to the pan
Stir-fry until slightly tender
Add more black pepper, soy sauce, oyster sauce, and a little water
Return the beef to the pan
Toss everything together until the sauce coats the beef and vegetables
Cook until heated through and the sauce thickens
Serve hot with rice or noodles