Preheat the oven to 150°C.
Sauté the diced onions in 25g of butter until soft, then allow to cool.
Mix the onions, pork mince, sage, breadcrumbs, suet, and lemon zest in a bowl.
Butterfly the pork loin by making incisions until it lays flat.
Season the meat, spread the stuffing over it, and roll it into a sausage shape.
Tie the loin with string at several points and place it in a roasting tin.
Rub the remaining 50g of stuffing butter over the pork and season with sea salt.
Roast for five hours until the crackling is crisp and the meat is tender.
Boil the potatoes in salted water for 15 minutes or until tender.
Pass the potatoes through a ricer and mix with butter and cream until smooth.
Wilt the spring greens in a pan with butter and a splash of water.
Carve the pork and serve with mash, greens, and gravy.