Steps
1. Heat the vegetable oil in a large skillet or wok over medium-high heat.
2. Add the beef strips and cook until browned on all sides. Remove from the skillet and set aside.
3. In the same skillet, add the bell pepper, onion, and garlic. Stir fry for 2-3 minutes, until the vegetables are tender-crisp.
4. In a small bowl, mix together the soy sauce, oyster sauce, black pepper, and red pepper flakes (if using).
5. Return the beef to the skillet and pour the sauce over it. Stir fry for an additional 2-3 minutes, until the beef is cooked through and coated in the sauce.
6. Season with salt to taste.
7. Serve hot with steamed rice or noodles.