Preheat the oven to 180°C.
Finely chop the mushrooms, onion and sage leaves.
Heat the butter and half of the oil in a frying pan. Add the mushroom & onion and cook until soft and the liquid has evaporated.
Turn off the heat. Let it cool. Mix in the parmesan and sage. Season to taste.
Make a pocket in the pork fillet lengthwise from the side - rather like with pita bread - i.e. don't cut all the way through. Open it out and fill it with the stuffing.
Lay the prosciutto slices onto a chopping board, slightly overlapping each other and place the pork fillet on them. Wrap the pork around with the ham completely.
Heat the remaining oil in the frying pan and cook the pork (seam side down) for a couple of minutes, then turn over and cook on each side until the ham is lightly coloured.
In the meantime core the apples and quarter them. Brush them with some honey/maple syrup if you like.
Place the ham in a roasting dish, scatter the apples, drizzle with some oil and grind some fresh peppercorns all over.
Garnish with a couple more sage leaves.
Bake for 30 minutes, then rest for at least 5 minutes before cutting it into slices. Serve it with the cooking juices drizzled over.
Serve with a mash.