In a medium bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for 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 2-3 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 sliced bell pepper, onion, garlic, and ginger. Stir-fry for 3-4 minutes until the vegetables are tender.
Return the beef to the skillet. Add the oyster sauce, sugar, beef broth, and sesame oil. Stir well to combine and cook for an additional 2-3 minutes until the sauce thickens slightly. Season with salt and pepper to taste.
Serve hot over cooked rice.