Remove fat from meat. If necessary, cut meat to fit into a 4-to-5-quart slow cooker. Place leeks in cooker. Place meat on leeks. Sprinkle with capers, marjoram, salt, and pepper. Add beef broth.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
Remove meat 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 the liquid in cooker. Cover and cook for 15 minutes more or until thickened. Spoon some of the sauce over meat. Pass the remaining sauce.