Choose a good leg of lamb with a thin layer of fat. With the point of a sharp knife or skewer, make deep holes all over the lamb, about 2.5cm apart. It is a good idea not to do this on the underside of the joint, in case somebody insists on eating their lamb unflavoured. Divide the rosemary sprigs into tufts of three or four leaves together.
Peel the garlic cloves and cut them into litt