Heat a skillet over medium heat and brown the beef stew meat on all sides. This step builds flavor before the long cook time.
Transfer the browned beef to the slow cooker. Add the pearl barley, carrots, celery, onion, beef broth, diced tomatoes, and dried thyme.
Stir the ingredients together and season with salt and black pepper.
Cover and cook on low for 7 to 8 hours until the beef is tender and the barley is soft.
Taste and adjust seasoning if needed. Stir in the fresh parsley just before serving.