In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for 15 minutes.
In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat. Add the marinated steak and stir-fry for about 3-4 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 onion and bell peppers, and stir-fry for about 5 minutes until they are tender-crisp.
Add the minced garlic and ginger to the skillet, cooking for an additional 1 minute until fragrant.
Return the cooked steak to the skillet. Pour in the beef broth, oyster sauce, and sesame oil. Stir everything together and let it simmer for 2-3 minutes until the sauce thickens slightly. Season with salt and pepper to taste.
Serve the pepper steak over cooked rice.