Add the beef stew meat to the slow cooker.
Stir in the rinsed barley, sliced carrots, chopped celery, diced onion, and minced garlic.
Pour in the beef broth, then add the tomato paste, Worcestershire sauce, dried thyme, bay leaves, salt, and black pepper. Mix everything until the tomato paste is distributed.
Cover the slow cooker and cook on low for 8 hours or on high for 4 to 5 hours, until the beef is fork-tender.
Remove and discard the bay leaves before serving.