Cut roast in half. Place in a 4-qt. slow cooker. Combine the water with salad dressing and au jus mixes; pour over meat. Top with onion and chilies., Cover and cook on low for 9-11 hours or until meat is tender. Remove meat. When cool enough to handle, shred meat with two forks. Skim fat from cooking juices. Return meat to slow cooker; heat through. Serve on rolls. Freeze option: Freeze individual portions of cooled meat mixture and juices freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally and adding a little water if necessary. Serve on rolls.