In a large bowl, combine the sliced beef, soy sauce, and cornstarch. Mix well and let it marinate for 10 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat. Add the marinated beef and cook until browned and cooked through. Remove from the pan and set aside.
In the same pan, add the remaining tablespoon of oil. Add the bell peppers, onion, garlic, and ginger. Stir-fry until the vegetables are tender-crisp.
Return the beef to the pan. Add oyster sauce, rice vinegar, beef broth, sugar, and black pepper. Stir well to combine.
In a small bowl, mix the cornstarch with water to form a slurry. Pour this slurry into the pan and stir until the sauce thickens.
Serve hot over cooked rice and enjoy!
Notes
For best results, make sure to slice the beef very thinly so it cooks quickly and remains tender.