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 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 minced garlic and grated ginger, and stir-fry for about 30 seconds until fragrant.
Add the chopped bok choy and sliced red bell pepper to the skillet. Stir-fry for 3-4 minutes until the vegetables are tender but still crisp.
Return the cooked beef to the skillet and add the oyster sauce and sesame oil. Stir everything together and cook for an additional 2 minutes. Season with salt and pepper to taste.
Serve the stir-fried beef and bok choy over cooked rice.