In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well 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 garlic and ginger, stir-frying for about 30 seconds until fragrant.

Add the chopped bok choy and sliced bell pepper to the skillet. Stir-fry for about 3-4 minutes until the vegetables are tender but still crisp.

Return the cooked beef to the skillet. Add the oyster sauce, sesame oil, salt, and pepper. Stir everything together and cook for an additional 2 minutes until heated through.

Serve the stir-fry over cooked rice or noodles.