For the pork belly, mix the salt, lemongrass, spices, garlic, ginger and red chillies together in a small bowl, then rub the marinade into the flesh side of the pork belly.
Cover with cling film and marinate for at least two hours, or up to 12 hours, in the fridge.
To cook the pork belly sous-vide, preheat the water bath to 85C/185F. Vacuum-pack the pork belly with the marinade and cook in the water bath for five hours. Remove from the water bath, remove the pouch and set aside to cool. Strain the cooking liquor through a fine sieve and set aside.
Press the pork belly between two baking trays with a weight on top to flatten the skin and place in the fridge for 1-2 hours.
(To cook the pork belly in a conventional oven, preheat the oven to 150C/300F/Gas 2. Place the pork belly skin-side up in a large ovenproof roasting dish. Add 200ml/7fl oz of water and place over a high heat on the hob until the water boils. Cover with a lid or foil and place in the oven for 2½ hours.Remove from the oven and check that the meat is cooked – if you are able to push the handle of a tablespoon through the belly, it is done. Alternatively, use a meat thermometer – the inside of the meat should have reached about 85C/185F. Strain the cooking liquor through a fine sieve and set aside. Allow the meat to cool, then press between two baking trays with a weight on top to flatten the skin and place in the fridge to chill for 1-2 hours.)
To finish the pork, heat the oven to 180C/350F/Gas 4.
Heat the rapeseed oil in an ovenproof frying pan over a medium heat and gently cook the pork, skin-side down for 10-15 minutes to crisp the skin, then transfer to the oven for 10 minutes to warm through.
Meanwhile, place the strained cooking liquor in a small saucepan with 200ml/7fl oz water and bring to the boil.
Season to taste with salt and freshly ground black pepper and a little soy sauce and sesame oil.
Meanwhile, for the cabbage, place the cabbage with the butter, three tablespoons water, salt and pepper into a large lidded saucepan.
Cover with a lid and cook over a medium heat for 5-7 minutes, or until the cabbage is tender.
Strain off any excess liquid and set aside with the lid on to keep warm.
Serve the pork sitting on a bed of cabbage with the hot cooking juices poured over, or put the whole belly, the cabbage and the juices in the roasting pot and place in the middle of the table for guests to help themselves.