Add the lean beef stew meat, carrots, celery, onion, and garlic to the slow cooker.
Stir in the rinsed pearl barley, beef broth, diced tomatoes with their juice, dried thyme, bay leaves, salt, and black pepper.
Cover and cook on low for 7 to 8 hours or on high for 4 hours, until the beef is tender and the barley is cooked through.
Remove and discard the bay leaves.
Stir in the chopped fresh parsley just before serving.