Heat the oven to 220c/fan 200c/gas 7. On a board, strip the rosemary from its stems, combining the leaves with 1 tbsp salt and a heavy bombardment of black pepper. Chop this all together until you are left with a wonderful lichen-coloured rosemary-salt mix.
Take the lamb shoulder and, with the tip of a very sharp knife, lightly score the top side with close parallel lines, making sure you do not cut into the meat. Rub in your salt and rosemary mix, really massaging it into the scoring you have made, and not forgetting the underside of the joint. Leave the lamb to one side.