Place the beef stew meat in the bottom of the slow cooker.
Add the pearl barley, carrots, celery, onion, and garlic on top of the beef.
Pour in the beef broth and the can of diced tomatoes, including the juice.
Sprinkle the dried thyme, salt, and black pepper over the mixture, then tuck in the bay leaves.
Cover the slow cooker and cook on low for 8 hours until the beef is fork-tender and the barley is cooked through.
Remove the bay leaves before serving.