Heat the oil in a large wok or skillet over high heat.
Add the beef to the wok and cook until it turns brown but still pink inside. Transfer to a separate plate.
Add garlic, ginger, and onions, and cook for 2 minutes. Stir in the peppers, green beans, and mushrooms, and cook for 3-4 minutes.
In a medium bowl, whisk together soy sauce, oyster sauce, Chinese cooking wine, brown sugar, cornstarch, and beef stock. Stir in the sauce mixture, stirring continuously, until the sauce has thickened.
Return the cooked beef and black pepper to the wok and toss to coat well. Serve immediately.