Place the beef stew meat, pearl barley, onion, carrots, and celery into the slow cooker.
Pour the beef broth over the ingredients.
Add the thyme sprigs along with the salt and black pepper.
Stir everything together so the barley and vegetables are mostly submerged.
Cover and cook on low for 8 hours, until the beef is fork-tender and the barley is soft.
Remove the thyme sprigs before ladling the soup into bowls.