Divide your dough in two and roll each piece out to a 1/2cm thickness. Cut one piece into a rectangle and lay it on a greased baking tray.
Cut the other piece into a rectangle of the same size then cut it into 6 long, thin, lengthwise strips.
Dress your bottom pizza 'base' with mustard, onion and cheese. Lay down your strips on top. Weave the frankfurters through your pizza strips. Fold 3 back on themselves and lay a frankfurter on top of the other 3. Fold the first 3 back and repeat until all the franks are in place. Brush with butter and sprinkle with coarse salt.