Rub the leg of lamb with the salt and then layer on the Shawarma paste, then leave to marinate for as long as you can, overnight is best.
Preheat the oven to 240C.
Slice two of the onions into half-moon segments then layer across the bottom of a roasting dish. Place the butterflied lamb over the top of the onions and put into a hot oven, roasting for 25 minutes until the lamb has browned nicely.
Remove the lamb from the oven and add 500ml of water to the dish, cover with foil and place back into the oven, turning the temperature down to 150C. Cook for a further 2 hours, checking from time to time that it hasn’t dried out, add a small amount of water if you need too. The lab should be tender and almost falling apart.
Remove the lamb from the oven and leave to rest in the covered dish.
In a pan melt the ghee and add the last onion, finely diced, with a few pinches of salt to draw out the liquid from the onion. Cook until softened for around 10-12 minutes.
Rinse the rice in a sieve under cold water and add to the pan along with the hot stock and finely sliced preserved lemon. Bring the rice to the boil then turn the heat down to low and cover the water with a circle of baking parchment that neatly fits the top of the pan, seal with the lid. Cook on low for around 20 minutes, until all the water has been absorbed.
Once the rice is cooked, roughly chop the herbs and mix through the warm rice. Decant the rice onto a platter. Take the lamb that has been resting, slice and scatter over the rice. Spoon over the juices from the pan and add the soft roasted onions over the top of the lamb. Finally, add a few fresh slices of preserved lemon for a citrusy freshness.