Heat half the oil in a large frying pan. Core and cut the apples into wedges, then cook for about 10 mins until caramelised and softened. Remove from the pan and set aside. Heat the remaining oil. Fry the pork on each side for 2 mins. Add the stock and mustard to the pan, then bubble for 5 mins or until the pork is cooked through. Return the apples to the pan with the sage and cook for 1 min more.
Remove from the heat and stir in the crème fraîche and some seasoning.