Wash and peel the roots and cut into 1 and one-half inch squares. Braise the meat until light brown, add the onions and cook 5 minutes more. Add the roots, the carrots, salt, pepper and the water, and cook on low fire for 1 hour or until the celery root is tender. Beat the egg and lemon juice, add some of the stock to warm up the juice, then pour this sauce over the stew slowly, mixing well. Serve at once.