Heat the oil in a large frying pan over a medium heat. Cook the sausages for 10-15 minutes, turning regularly, until cooked through with no pink remaining. Remove from the pan and set aside, then add the onions and fry for 5 minutes until soft and golden.
Return the sausages to the pan with the onions and add the apple. Pour in the cider and simmer until the liquid is reduced by half.
Stir in the apple sauce, mustard and crème fraîche and cook for 5 minutes.
Take the pan off the heat and stir in the chopped parsley. Serve with mashed potato.