Preheat oven to 475˚F. Pat the skin of the meat dry and season the flesh side with pepper and half the salt. Sprinkle the sage leaves on the bottom of a metal baking dish (do not use a glass baking dish as it might shatter when you add the milk) and put the pork on top, skin side up. Season the top with the remaining salt.
Roast for 20-30 minutes at 475˚F until the skin is starting to blister and crackle. Watch closely for burning.
Pour the milk around the meat to come about half to two thirds of the way up the sides of the pork. Reduce the heat to 325˚F and roast for a further 1½ hours or until the meat is meltingly tender. Check the level of liquid during cooking and if it has evaporated add a little more to the pan.
Remove the pork from the oven, lift it out of the dish and allow it to cool. Discard the liquids (they will break into curds).
For easy cutting, place the meat flesh side up on a chopping board and use a heavy, sharp knife to cut it into slices about 3-4cm thick. Serve warm or at room temperature.