To make the stuffing, melt the butter in a large non-stick frying pan and add the onion and mushrooms. Fry over a medium-high heat for 4-5 minutes, stirring regularly until all the liquid has evaporated. Remove from the heat and leave to cool for 10 minutes. Stir in the Parmesan and sage and season well with salt and pepper.
Slice the pork fillet along its length without cutting all the way through. Open out and spoon the stuffing inside. Lay the Parma ham on a clean surface in slightly overlapping slices and place the stuffed pork fillet on top. Wrap the ham around the pork so it is completely covered.
Preheat the oven to 200C/fan 180C/Gas 6. Heat the olive oil in a large non-stick frying pan or sauté pan and add the pork, seam side down. Cook for a minute then turn onto another side. Continue cooking and turning for 3-4 minutes more or until the Parma ham is lightly coloured on all sides. Transfer to a baking tray, scatter the apples around the pork and bake for 30 minutes. Leave the pork to rest for 5 minutes, then cut into thick slices and serve with the roasted apples. Spoon any juices from the baking tray over the pork and garnish with extra sage leaves if you like. Serve with some leafy greens.
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