Prep the Beef: Toss the beef slices with cornstarch and a pinch of black pepper.
Cook the Beef: Heat 1 tablespoon of oil in a large skillet or wok over high heat. Stir-fry the beef until browned but not fully cooked. Remove and set aside.
Sauté Vegetables: Add the remaining oil, then stir-fry the bell peppers, onions, and garlic until slightly tender but still crisp.
Make the Sauce: Stir in soy sauce, oyster sauce (if using), beef broth, and sugar. Bring to a simmer.
Combine & Finish: Return the beef to the pan and toss everything together until the sauce thickens slightly and coats the beef and vegetables. Adjust seasoning with black pepper.
Serve: Serve hot over steamed rice or noodles.
