Preheat your oven to 200ºC/400ºF/gas 6.
Place your pork loin in front of you and score across the skin with a sharp knife, or a Stanley knife, about 1cm/½ inch deep and about 1cm/½ inch apart. Pound up the rosemary and fennel seeds with a tablespoon of salt – bash the mixture up until really fine and then rub it into all the score marks on the pork. Remove the crusts from the bread and slice it up. I like to toast the bread in a toaster or on a griddle until lightly golden, as this gives the stuffing a really fantastic smoky flavour.
While the bread is toasting, slowly fry the onions, garlic, sage and pine nuts in a little olive oil for 10 minutes until the onions are sweet and soft. Season with salt and pepper, add the balsamic vinegar and put the mixture in a bowl. Rip your bread into pieces and add to the bowl. Squash everything together, really squeezing the onions into the bread. Have a taste – it may need a little more seasoning. Put to one side and allow to cool.
Insert your knife into the eye meat of the pork loin and make a cavity for your stuffing. Pack in the stuffing, then roll the pork over and tie it with a few pieces of string. Place the pork on a roasting tray and cook in the preheated oven for just over an hour until crisp and golden.