In a medium 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 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 3-5 minutes until they are tender-crisp.
Add the minced garlic and ginger to the vegetables, and stir-fry 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 cook for another 2-3 minutes until heated through. Season with salt and pepper to taste.
Serve the pepper steak over cooked rice.