Prepare the Beef. Slice the beef thinly against the grain for maximum tenderness. In a bowl, combine the sliced beef with soy sauce, minced garlic, ground black pepper, and cornstarch. Toss until the beef is evenly coated, then set aside to marinate for 10-15 minutes.
Sauté the Vegetables. Heat vegetable oil in a large skillet or wok over medium-high heat. Add the sliced onions and bell peppers to the pan and sauté until they are tender-crisp, about 3-4 minutes. Remove the vegetables from the pan and set aside.
Cook the Beef. In the same skillet, add the marinated beef slices in a single layer. Cook undisturbed for 1-2 minutes to allow them to sear and develop a golden crust. Stir-fry until the beef is cooked to your desired doneness, then return the cooked vegetables to the pan.
Thicken the Sauce. In a small bowl, mix together soy sauce, beef broth, and cornstarch until smooth. Pour the mixture into the skillet with the beef and vegetables. Cook, stirring constantly, until the sauce thickens and coats the beef and vegetables evenly.
Serve and Enjoy. Garnish the Pepper Steak with freshly chopped green onions or sesame seeds for extra flavor and visual appeal. Serve hot over steamed rice or noodles for a complete and satisfying meal.