Slice the flank steak against the grain into thin strips.

Slice the bell peppers and onion, and mince the garlic and ginger.

In a small bowl, mix soy sauce, oyster sauce, cornstarch, and black pepper. Set aside.

Heat vegetable oil in a wok over high heat. Add the beef strips and stir-fry until browned, about 3-4 minutes. Remove from the wok and set aside.

In the same wok, add garlic, ginger, onion, and bell peppers. Stir-fry for 3-4 minutes until vegetables are tender-crisp.

Return the beef to the wok, add the sauce, and stir well to combine. Cook for an additional 2 minutes until the sauce thickens.

Serve hot over rice or noodles.