In a medium-size container, combine beef broth with tomato puree, Worcestershire sauce and ground cumin. Pour the mixture over the meat. Add chopped tomatoes, sautéed onions, pepper and cilantro. Cover the slow cooker and set to cook on low for 7-8 hours or until the meat is tender and can be pulled apart easily. Turn off the slow cooker and shred the meat with the help of 2 forks.