In a mixing bowl, combine the beef slices, soy sauce, cornstarch, ground black pepper, and sesame oil. Mix well and let marinate for about 10 minutes.
While the beef is marinating, heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced onion and bell peppers to the skillet. Stir-fry for about 3-4 minutes until they are tender but still crisp. Remove them from the skillet and set aside.
In the same skillet, add the marinated beef. Stir-fry for about 4-5 minutes until the beef is browned and cooked through.
Add the minced garlic to the skillet and stir-fry for an additional minute until fragrant.
Return the cooked vegetables to the skillet with the beef. Add the oyster sauce, sugar, and beef broth. Stir well to combine everything.
Cook for another 2 minutes, allowing the sauce to thicken slightly.
Serve hot over cooked rice.