Heat the oil in a large frying pan and fry the pork slices in 2 batches until browned. Remove and set aside.
Add the onion and walnuts to the pan and fry for 3-4 minutes until softened. Stir in the flour and cook for 1 minute. Gradually blend in the cider, stock cube and mustard and bring to the boil, add the pork back to the pan with the apple and season. Transfer to a casserole dish, cover and bake for 1½ hours until pork is tender.
Stir in the crème fraiche and parsley and serve with rice or mashed potato and fresh vegetables.