Remove excess fat from meat. Sprinkle both sides of meat with salt and pepper. Cut meat into 4 serving-size pieces. Place fennel in 3 1/2- or 4-quart slow cooker. Place meat on top of fennel. Combine in a small bowl beef broth and mustard. Pour over meat.
Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours.
Remove meat and fennel to a serving platter, reserving cooking liquid. Cover meat with foil to keep warm.
If using low-heat setting, turn to high-heat setting. For sauce, combine in a small bowl half-and-half and cornstarch. Stir into cooking liquid. Cover and cook 15 minutes more or until sauce is thickened. Serve meat and fennel with sauce.