1. Cut the lamb into strips three centimetres wide and ideally about 10 centimetres long,then mix with the remaining ingredients and the spices.
2. Take a sheet of foil,about 30 centimetres by 50 centimetres. In the centre,arrange the lamb in a single layer about two to three centimetres high,by standing the slices upright and pressing them firmly and closely together,to form a reasonably solid block. It's slightly tricky but try to keep the strips of meat on their edges,even though they're different sizes. Then,fold the foil up and over tightly to form a parcel,so the meat is held in place.
3. Carefully push six to eight skewers horizontally through the parcel so the meat is secured,snip the skewer points off to fit into the pan,and leave the meat to firm in the fridge for an hour or more (overnight is fine).
4. To cook,lightly oil a hot frying pan. Tear the foil off the meat,slip the shawarma slab into the pan and fry over a high heat until very brown underneath. Flip,and cook the other side.
5.Serve on a cutting board and slice through the strips of meat with a sharp knife,discarding the skewers.
Tip: Use bamboo skewers to hold lots of meat together, like a fat,wide kebab,so it crisps perfectly in a frying pan,then slice it thinly to serve. Unlike individual kebabs,you keep more moisture inside the meat while getting an extra-crisp crust.