Cut the pork into small pieces, discarding any excess fat. Season the flour with a little salt and use to coat the meat.
Melt the butter with the oil in a shallow, flameproof casserole and gently fry the pork in batches until lightly browned, draining each batch to a plate.
Add the onion and leek to the casserole and fry gently for 5 minutes. Return the meat to the pan, along with the cider, sage and mustard. Bring just to the boil, then cover with a lid, reduce the heat and cook on the lowest setting for 30 minutes.
Stir the crème fraîche into the sauce and scatter the pear slices on top. Arrange the sweet potato slices in overlapping layers on top, putting the end pieces underneath and keeping the best slices for the top layer. Brush with the chilli oil and sprinkle with salt.
Place in a preheated oven, 160°C (325°F), Gas Mark 3, for 45 minutes or until the potatoes are tender and lightly browned. Scatter with the chopped parsley.