Preheat the grill to high. Wash the potatoes, chop into 3cm chunks, then cook in a large pan of boiling salted water with the lid on for 12 minutes, or until tender. Meanwhile, put a shallow casserole pan on a high heat. Season the pork with sea salt and black pepper, then place in the pan with 1 tablespoon of olive oil and sear for 3 minutes, turning regularly, while you pick the sage leaves. Remove the pork to a plate, toss the sage leaves into the fat in the pan for just 5 seconds, then scoop on to a plate, leaving the pan off the heat to use again.
Drain the spuds, tip into the casserole pan, grate over half the cheese, add 1 tablespoon of extra virgin olive oil and mash well, loosening with a splash of water, if needed. Taste, season to perfection, and spread out to the edges. Grate over the remaining cheese, sit the pork on top, then gratinate under the grill for 10 minutes. Lay the prosciutto around the pork in waves, sprinkle over the crispy sage, then grill for 2 more minutes, or until the pork is cooked to your liking. Rest for 2 minutes, then slice the pork and dish up.