If you have a good butcher, ask him for the rib or rump end of the pork loin ? it?s more evenly sized, making it easier to cook. Ask him to leave the skin on and to score it across with lines about 5mm/1/4 in. apart and then to take it off the bone. Ask him to chop the bones up for you and take them home to use for your gravy.
Lay out your pork on a board and rub some salt and 1 teaspoon chopped rosemary into the scored lines, trying to get this into every bit by pushing and rubbing in. In a pestle and mortar smash up the fennel seeds, then the garlic and remaining chopped rosemary, and rub this into the meat ? not the skin, or it will burn. Place in a large roasting tray with the balsamic vinegar, bay and olive oil. Leave for about 1/2 hour to marinate.
Meanwhile, preheat your oven to its highest temperature and brown the bones. Rub the skin of the pork with lots of sea salt ? this will help puff it up and dry it out. Place the pork directly on the bars at the top of the oven. Finally add the browned bones and vegetables to the leftover balsamic marinade, add 570ml, 1 pint water and put into the oven directly under the pork. As the pork cooks all the goodness drips from it into the tray. This liquid will then become your gravy. You also get quite charred bar marks on the base of the pork.
The pork will take about 1 hour to cook. After 20 minutes turn the temperature down to 220C/425F/Gas 7. Once the pork is cooked, remove it from the oven on the rack and place on a piece of foil to save any juices. Allow to rest for at least 10 minutes. Finish off any vegetables that you are going to serve with it and make a gravy out of the juices in the tray which was underneath the pork.
Put the bones, the liquid and the vegetables into a large pan. Add some water to the tray that contained the bones and vegetables, as there will be some Marmite-like, sticky stuff on the bottom to the tray which is very tasty. Reboil the water, scrape off all the goodness from the bottom of the tray and then pour everything into the pan. Bring to the boil, shaking occasionally, remove any oil, grease or scum from the top, then pass the contents through a sieve, discarding all the vegetables and bones. You can reduce and then correct the seasoning, to taste.