Slice the sirloin steak into thin strips and season with 1 tablespoon of cornstarch and freshly ground black pepper.
Heat vegetable oil in a large skillet over medium-high heat. Add steak strips and cook until browned, about 3-4 minutes. Remove from skillet and set aside.
In the same skillet, add minced garlic and sliced bell peppers. Stir-fry for about 3 minutes until peppers are tender-crisp.
Mix beef broth, soy sauce, and remaining 1 tablespoon cornstarch in a bowl. Pour into skillet and cook, stirring constantly, until sauce thickens.
Return cooked steak to the skillet and toss to coat with the sauce. Cook for an additional 2 minutes until heated through.
Serve hot over rice or noodles.
Notes
You can adjust the black pepper quantity to make the dish spicier or milder according to your taste.