Preheat the oven to 150C/300F/Gas 2.
Put the onions in a food processor and process until they are finely chopped.
Heat a frying pan until medium hot, add 25g/1oz of the butter and, when it’s foaming, add the chopped onion and cook for five minutes until just softened.
Put the sage, chestnuts and breadcrumbs into a bowl with the onions, season with plenty of salt and black pepper, then mix well to combine.
Lay the pork, flesh-side up, on a chopping board with the belly flap away from you.
With a sharp knife, make an incision in the eye of the loin half way down running parallel to the board, cutting almost all the way through. Open the loin up and repeat a few times, cutting so that the loin opens up and it is as flat as the belly part of the pork.
Season with salt and black pepper then spoon the stuffing onto the loin and spread out along the loin, then roll up into a large sausage.
Secure with cook's string at 5-6 points along the loin then transfer to a roasting tin.
Rub the rest of the butter over the top of the pork and season with plenty of sea salt.
Put into the oven and roast for five hours. The crackling should be crisp and the pork very tender.
For the dauphinoise potatoes, when the pork has cooked for about 3¾ hours start preparing the potatoes. Rub an ovenproof dish with the garlic, then rub about a teaspoon of the butter around the inside and place the potatoes in the dish in layers, overlapping a little, until the dish is just full. Season with salt, pepper and nutmeg between each layer.
Pour the cream and milk over the potatoes to cover, then season with salt and black pepper and dot with the remaining butter. Cover tightly with aluminium foil and place in the oven when the pork has been roasting for four hours.
Once the potatoes have been cooking for 30 minutes, turn the oven up to 200C/400F/Gas 6, remove the foil and cook for another 30 minutes, or until the potatoes are tender and the pork crackling is crisp.
Remove the potatoes and pork from the oven and let the pork rest for 30 minutes (leave the potatoes in longer if they need more cooking while the pork is resting).
To serve, carve the pork into slices and serve with the roasting juices and dauphinoise potatoes.