In a medium bowl, combine the sliced flank steak, soy sauce, and cornstarch. Toss to coat the beef evenly and let it marinate for about 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the chopped cabbage, sliced bell pepper, and onion. Stir-fry for about 5 minutes until the vegetables are tender-crisp.
Add the minced garlic and ginger to the skillet and stir-fry for an additional 1-2 minutes until fragrant.
Return the cooked beef to the skillet. Add black pepper, oyster sauce, rice vinegar, and salt to taste. Stir everything together and cook for another 2-3 minutes until heated through.
Serve hot over cooked rice or noodles.