Place brisket, carrots, onion, celery, bay leaf, salt, pepper, and water in a Dutch oven. Bring to a boil; reduce heat. Cover and simmer 3 hours or until meat is very tender.
Transfer brisket to a warm serving platter; discard broth. Cut brisket diagonally across grain into thin slices; pour Horseradish Sauce over top. Sprinkle with parsley, if desired.