In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for about 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated steak in a single layer and cook for about 2-3 minutes until browned. Remove the steak from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced bell pepper and onion, 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 an additional 1 minute until fragrant.
Return the cooked steak to the skillet. Pour in the beef broth and oyster sauce, stirring to combine. Cook for another 2-3 minutes until the sauce thickens slightly.
Drizzle with sesame oil and season with salt and pepper to taste. Serve hot over cooked rice or noodles.