Layer the potatoes, carrots, onion and celery in a 5-qt. slow cooker. Place flour in a large shallow dish. Add stew meat; turn to coat evenly. In a large skillet, brown meat in oil in batches. Place over vegetables., In a large bowl, combine the tomatoes, broth, mustard, salt, pepper, thyme and, if desired, browning sauce. Pour over beef. Cover and cook on low for 7-8 hours, or until the meat and vegetables are tender. If desired, sprinkle with fresh thyme before serving.