In a frying pan, heat up half of the butter and then fry the onion on a medium heat. Slice the hotdog sausage open, being careful not to cut it completely in half. Heat the remaining butter in a separate pan and fry the sausage for a couple of minutes, until golden brown and heated through.
Open the roll and place the sausage in it, with the onion on top, followed by some mustard and some sauerkraut.
For an alternative you can serve the hotdog with chilli con carne on top instead of the mustard and sauerkraut.