Heat a little of the oil in a frying pan over medium-high heat. Add beef cubes in batches and sear until golden brown. Set aside.
Add remaining oil to pan and cook onion and garlic for 3-4 minutes, until tender.
Pour the beef stock and water into the bowl of the slow cooker. Add barley, carrots, celery, onion mixture, beef cubes and thyme sprigs. Cook on low setting for 8 hours. Season to taste, fold in parsley and accompany with some crusty bread and green beans.