Preheat the oven to 170°C/325°F/gas mark 3. Put the pork fillets, spaced apart, between 2 large sheets of greaseproof paper or cling film that have been smeared with oil. Then beat with a rolling pin until the meat has spread out thinly — it should be about 3mm/⅛in thick. Remove the paper, lay the fillets out and grate the nutmeg all over them. Finely chop the sage and rosemary and sprinkle evenly over the fillets, followed by salt and black pepper. Slice the cheese as thinly as possible and lay it evenly on top of the herbs. Roll each fillet up from the shortest end, folding over the edges of the meat to enclose the cheese.
Carefully put the rolls into a roasting pan, dot with butter, cover the pan with foil and cook for 1-1½ hours, until the cheese has just begun to ooze out and the juices run clear when a skewer is inserted in the meat. Transfer the rolls to a serving plate. Spoon the soured cream over them, sprinkling generously with Parmesan. To serve, cut the rolls across in round slices, using a sharp knife.