Preheat the oven to 200ºC/400ºF/gas 6. Score the pork skin with a sharp knife at 1cm intervals, and roughly 1cm deep. Pick the rosemary leaves into a pestle and mortar, add the fennel seeds and 1 tablespoon of sea salt, then bash to a dry paste. Massage into the score marks on the pork. Slice up the bread, and toast until golden. Meanwhile, peel and finely slice the onions and garlic, and pick and roughly tear the sage leaves. Drizzle 1 tablespoon of olive oil into a pan over a medium heat, add the onions, garlic, sage and pine nuts and cook for 10 minutes, or until sweet and soft. Season with salt and black pepper, add the balsamic vinegar, then transfer the mixture to a bowl. Rip the toasted bread into the bowl, then scrunch everything together really well. Have a taste and adjust the seasoning, if needed. Leave to cool. Insert a knife into the eye meat of the pork loin and make a pocket for the stuffing. Pack in the stuffing, then roll the pork over and tie it with butcher's string to secure. Place the pork in a roasting tray and roast for 1 hour, or until golden, crisp and perfectly cooked.