Put the potato pieces, rosemary, olive oil, pepper and salt in a bowl, mix well, place one at a time on a baking sheet with sufficient space between them and place in the oven preheated to 200 ° C. Depending on the thickness of the pieces, they take between 15 and 25 minutes to become crispy.
During this time, rinse the schnitzel, pat dry and gently flatten it with a meat tenderizer. Sprinkle with salt and pepper on both sides and then cut in half. Place a suitable slice of Parma ham on each piece. Place 1 - 2 sage leaves on top of these. Fold the schnitzel together and use a toothpick to pin the two halves together. Turn the schnitzel in a little flour on a plate.
Heat the ghee in a pan and carefully place the schnitzel in it. Fry for 3 - 4 minutes on both sides. Place on preheated plates. Add the stock and vinegar to the pan. Loosen the roast set and let it boil down a little. Add the butter and beat until frothy with a whisk. Season to taste with salt and pepper.
Put the potatoes on the plates with the meat. Pour a little sauce over the meat and serve immediately.