Start by making the stuffing: In a large bowl, place the butternut squash, hazelnuts, red onion, sage, breadcrumbs, and melted butter. Season with salt and black pepper and mix well.
Butterfly the pork loin roast: Make a slit down its length, cutting just deep enough so that the loin opens up to lie flat like a book. Do not cut all the way through.
Spoon the stuffing mixture onto the meat and spread evenly.
Close up the loin and, using pieces of butcher’s twine, tie at even intervals so that it assumes its original shape. Push in any stuffing that escapes from the ends.
Arrange the whole shallots on a baking sheet, season with sea salt and freshly ground black pepper.
Rub the joint all over with salt and black pepper and place on top of the shallots on the baking tray. Roast for 25 minutes, then reduce the heat to 350°F and cook for another 20 minutes per 500 grams.