Remove excess fat from meat. Cut meat into 1-inch pieces. Sprinkle meat with pepper. In a large nonstick skillet cook the meat, (half at a time), in hot oil over medium heat until brown. Drain off fat. Place meat in a 4- to 5-quart slow cooker. Stir in beef broth, spaghetti sauce, and fennel.
Cover and cook on low-heat setting for 10 to 11 hours or on high-heat setting for 5 to 5 1/2 hours.
Stir in zucchini. Turn off heat. Cover and let stand for 15 minutes before serving. Top each serving with Parmesan cheese (optional).