Instructions:

1. Prepare the beef:
In a bowl, combine the sliced beef, salt, pepper, and cornstarch. Mix well to coat the beef evenly.

2. Stir-fry the beef:
Heat 1 tbsp vegetable oil in a wok or large skillet over high heat. Add the beef and stir-fry until browned on all sides. Remove the beef from the wok and set aside.

3. Cook the vegetables:
Add the remaining 1 tbsp vegetable oil to the wok. Add the garlic and ginger and stir-fry until fragrant, about 30 seconds. Add the napa cabbage and stir-fry until it begins to wilt, about 2-3 minutes.

4. Combine and finish:
Add the cooked beef back to the wok. In a small bowl, combine the soy sauce, oyster sauce, rice vinegar, and sesame oil. Pour the sauce over the beef and cabbage and stir-fry until the sauce thickens and coats everything evenly, about 1-2 minutes.

5. Serve:
Garnish with green onions, if desired, and serve hot with rice or noodles.