Rub the salt into the outside of the roasting joint then place the joint in a shallow roasting dish. Peel the potatoes, cut them lengthways into four segments per potato then allow them to soak in fresh water for a few minutes to remove any excess starch. Roughly pat the potatoes dry and place them round the joint of pork in the roasting dish. Peel and slice the onions and spread these amongst the potatoes. Sprinkle the thyme over the top. Spread the butter around over the potatoes and add a few twists of fresh pepper.
Add a glass of water to the tin then place it at the top of a preheated hot oven at Mk 7 - 425ºF - 220ºC. for twenty minutes. After 20 minutes, reduce the heat to Mk 5 - 375ºF - 190ºC. and move the roasting tin down to the middle of the oven. Continue cooking for a further 40-50 minutes, occasionally basting the meat with the juices from the bottom of the tin. Turn the meat twice during cooking and the potatoes several times to ensure that they do not dry out. Remove the joint from the oven and check that juices run clear. Rest for 15-20 minutes before carving. Serve hot with fresh vegetables.