First, make the stuffing crust, as this will keep. Heat oven to 140C/120C fan/gas 1. Place the slices of bread on a baking sheet, drizzle with the rapeseed oil and dust with the cayenne pepper. Bake in the oven until the bread is completely dry and browned just a little – this will take about 20 mins. Remove from the oven and rub with the garlic cloves while still warm. Use a whole clove on each slice, or half if you have small slices. Leave the toasted slices to cool, then put them in a food processor and blitz to rough but not too big breadcrumbs. Empty the crumbs into a mixing bowl and add the dried sage and onion, the parsley and a good pinch of sea salt. Mix together, then set aside in an airtight container. Can be made up to 2 days ahead without the parsley (see tip, below left).
Turn down the oven to 120C/100C fan/ gas ½. Score the layer of fat on the pork with a sharp knife, working diagonally along the length, then score again in the opposite direction. Tie the loin with butcher’s string to help keep its shape and cook evenly. Heat the rapeseed oil in an ovenproof frying pan and add the pork, fat-side down. Sear and brown the top layer of fat, then turn the pork over and throw in the butter, garlic, rosemary, thyme, pepper and 2 tsp table salt. Once the butter has melted, add the lemon zest and juice to create a fragrant cooking liquor. Roll the pork around in the buttery juices and baste so that it is well coated.
Place the pan with the pork in the oven and set your timer for 1 hr 15 mins. Baste the meat with the buttery juices every 15 mins. At the end of the cooking time, test the thickest part of the pork with a digital thermometer – it should read 70C when ready. If the meat hasn’t reached the temperature, return it to the oven for 5 mins, then check again. Once the pork is cooked, remove from the oven and leave to rest for 10 mins. If you don’t have a digital thermometer, cook the pork for 1 hr 20 mins.
While the pork is resting, pour the milk for the polenta into a saucepan and bring to the boil. Whisk in the polenta and cook over a medium heat until thickened, following pack instructions – it needs to be the texture of creamy mash. If your polenta is too thick, add a little more milk to loosen it. When it is ready, remove from the heat and whisk in the butter, Parmesan, lemon zest, truffle oil, fresh truffle (if using), salt and the cayenne to taste. Pour into a serving bowl and sprinkle over the chives.
Untie the rested pork loin and give it a final baste with the cooking liquor. Tip the crust into a roasting tin, then roll the pork loin in it, making sure it gets a good covering. Thickly slice the pork and serve with the soft polenta, Romanesco or broccoli sprinkled with toasted hazelnuts, and the buttery juices.