To serve (optional)
IN THE KITCHEN
Bring the lamb to room temperature by removing form the fridge for about 30 minutes before cooking.
Sit the lamb on a chopping board, skin side up, and lightly score the skin with a sharp knife. Scatter the onions and garlic in the base of a large oven-proof dish or roasting tray that fits in your barbeque with the lid down, and pour over larger or ale. Sit the lamb on top of the onions.
Mix together the, lemon juice, cinnamon, cumin, coriander, chilli powder, turmeric, ginger, olive oil, a good pinch of salt and some black pepper. Slowly pour all over the top of the lamb.
ON THE BARBEQUE
Prepare the barbeque with the baffle in upright position, so it is burning at about 150°C for low and slow cooking.
Cover the dish or roasting tray with a lid or large piece of tin foil, securing tightly. Roast in the barbeque with the lid on for 3 ½ hours, then remove the foil. Increase the heat to around 250°C by increasing the air flow. Continue to roast for a further 30 minutes for the top of the lamb to colour.
When the lamb has cooked, transfer to a large warm serving plate and leave to rest for 20-30 minutes, loosely covered with foil.
Serve the meltingly tender lamb pulled apart into chunks, allowing it to soak up the juices on the bottom of the plate.