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 in a single layer and cook 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 onion and bell pepper, and stir-fry for about 3-4 minutes until they start to soften.
Add the minced garlic and ginger to the skillet, and stir-fry for another 30 seconds until fragrant.
Return the beef to the skillet, and pour in the beef broth and oyster sauce. Stir everything together and cook for an additional 2-3 minutes until the beef is cooked through and the sauce has thickened.
Drizzle with sesame oil, and season with salt and pepper to taste. Serve hot over cooked rice or noodles.