Heat the slow cooker. Heat the oil in a large frying pan, fry the pork medallions on each side for 2 mins until they pick up a little colour. Fry the onion for a few mins, then add the stock and mustard to the pan and stir. Tip the pork and sauce into the slow cooker.
Core and cut the apples into quarters, add them to the pot with the sage. Season with black pepper. Cook on Low for 4 hours or until the meat is tender, then stir in the crème fraîche.