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 sliced onion, bell pepper, and garlic. Stir-fry for about 3-5 minutes until the vegetables are tender.
Add the minced ginger and stir for another minute 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 heated through.
Drizzle with sesame oil and season with salt and pepper to taste.
Serve hot, garnished with chopped green onions.