Stir together water, onion soup mix, and au jus mix in a slow cooker. Add beef, cover, and cook on High for 5 hours, or until tender. When the beef is tender, shred the meat with two forks, and continue cooking on Low 1 to 2 hours.
To serve, open the hoagie buns, and spoon meat mixture into the center. Sprinkle with mozzarella cheese and serve.