Add roast and cook on high for approx 6 hours or until meat is easy to shred.
Remove meat to a platter and shred, removing any fat. Return shredded meat to slow cooker and add salt to taste, along with a little bit of the liquid to keep the meat juicy.
Pour the liquid from the slow cooker into a gravy strainer.
Pour off fat from the liquid and divide remaining aus jus into serving bowls for dipping.
To serve, slice rolls and top with meat and provolone cheese. Heat under the broiler until cheese is melted. Spread with horseradish (if desired) and serve with aus jus.