In a bowl, combine the sliced flank steak with soy sauce, oyster sauce, cornstarch, black pepper, and salt. Mix well and let marinate for at least 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 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 crushed red pepper flakes (if using) to the skillet, and cook for an additional minute until fragrant.
Return the cooked steak to the skillet, and pour in the beef broth. Stir everything together and cook for another 2-3 minutes until heated through and the sauce thickens slightly.
Serve the pepper steak over cooked rice.