Place the round steak between two sheets of plastic wrap. Using a meat mallet, pound round steak until thin. Prepare stove top stuffing according to package directions. Remove cover and set aside to cool. Meanwhile, heat oil in small sauté pan. Add celery, and onion if using, sauté until onion and celery are tender. About 10 minutes. Cool. Remove top sheet of plastic wrap. Spread stuffing on top of round steak, top with onion mixture. Carefully roll up round steak, tucking in sides if necessary. Secure with toothpicks or butchers twine. Carefully place in slow cooker. Pour beef broth over the top, season with salt and pepper. Cook on low for 6 hours.