If you want your leftover prime rib warmed up before adding it to the sandwich, I recommend first heating a rich au jus sauce made from the flavorful juices that cook off a roasted prime rib. If you made your prime rib on the smoker and don't have that option, even just warmed beef broth will work. Add your thin prime rib slices to the hot liquid for a minute or two until hot, then remove with tongs, letting the excess juices drip off before adding the meat to your sandwich.