Steam the potatoes for 20 minutes, and when just cool enough to handle, cut into 4cm slices. If using a whole truffle, slice as finely as possible, and if using shavings, dice finely. Heat the goose fat (if you’re a vegetarian, use a rich, fruity olive oil instead) in a large frying pan and, when hot, add the potatoes, truffle, parsley and seasoning, and fry over a medium heat for 5 minutes. Serve – making sure that you gather all the truffle pieces and parsley from the bottom of the pan. Drizzle with the truffle oil, if using.