Heat the olive oil in a skillet over medium heat. Add the beef stew meat and brown it on all sides for a few minutes. Transfer the meat to the slow cooker.
Add the diced onion, sliced carrots, cubed potatoes, and pearl barley to the slow cooker. Pour in the beef broth and Worcestershire sauce.
Stir in the dried thyme, bay leaves, salt, and black pepper. Cover and cook on low for 7 to 8 hours.
In the final 30 minutes, whisk the flour with a small amount of the hot broth from the slow cooker to form a smooth slurry. Stir the slurry back into the stew to thicken the gravy.
Remove the bay leaves before serving.