Heat a large skillet over medium-high heat, and add oil. Saute garlic for about 5 seconds, then add ground beef. Stir-fry until beef is browned. Stir in cabbage and bell pepper, and cook until vegetables are tender, and beef is cooked through. Add soy sauce and pepper.
In a small bowl, mix together cornstarch and beef broth. Stir in cornstarch mixture. Cook, stirring, until sauce has thickened.