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Using a hand blender or mini food processor, whizz marinade ingredients to a smooth paste, then season with salt. Lay the lamb in a ceramic dish and coat with the marinade. Cover in cling film and leave to marinate in the fridge for at least 4 hrs or preferably overnight.
If you are using a charcoal barbecue, wait for the coals to become white and glowing (light the barbecue about an hour before you want to cook the lamb). If using a gas barbecue, cook the lamb over medium heat. Lift lamb out of the marinade and barbecue, flesh-side down, for about 25 mins. Flip it over and give the fat side about 15 mins. When the lamb has a dark brown crust and is cooked to your liking, leave it to rest on a board for 15 mins. (If the weather forces you indoors, the lamb can be roasted at 220C/200C fan/gas 7 for about an hour for pink meat; the breads can be cooked on a very hot griddle).
To serve, carve the lamb into thick slices and scatter with the coriander and some of the Pickled red onion (see below). Serve the remaining onion in a dish alongside the raita and breads (see recipes, below).