Heat a drizzle of oil in a large skillet and brown the beef, seasoning with salt and pepper, and transfer to the bowl of a slow cooker.
Add the onion to the pan and cook for a few minutes, then transfer to the slow cooker along with the beef.
Pour about 1/2 cup water into the pan and swirl it around to loosen any browned bits; add to the slow cooker along with the stock, Worcestershire sauce, tomato paste and thyme.
Cover and cook on low for 4-6 hours, or until the meat is very tender. (Alternatively, braise in the oven at 325˚F for 2 1/2-3 hours.)
Add the celery, carrots and barley, cover and continue to cook for another hour, or until the vegetables and barley are tender.
Pull the beef into smaller pieces with a fork, right in the pot, before serving.