Not sure if liver counts as offal or not but per my recollection it's simply liver (usually), oatmeal, some extra fat, a little salt, onion, a lot of ordinary black pepper. If sausage type it goes in a natural skin casing (intestine) or may be the plasticy stuff salami comes in is for the salami-size kind. Boiling haggis is the same stuff stuffed in a sheep stomach.