Preheat the oven to 160C/325F/Gas 3. Season the pork on both sides with sea salt and freshly ground black pepper and set aside. Heat the butter in a large frying pan and add the apple pieces.
To make the stuffed loin of pork, place pork, silverskin side down, vertically in front of you. With a large knife, make an incision along the length of the loin 2cm/3/4in from the edge.