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Mix the butter and oil in a bowl, then stir in the spices, thyme, garlic, lemon zest and juice. Add the harissa, 1 tsp salt and plenty of freshly ground black pepper, and mix well.
Put the butterflied lamb (see step-by-step guide) in a large shallow dish and spoon over the marinade. Using your hands, rub it all over the meat. Cover loosely with foil or cling film and leave to marinate for at least 2 hrs, or overnight in the fridge.
Light the barbecue, adding plenty of coals. When it is ready, add the lamb, fat-side down, and cook on a fairly high heat for 5 mins until well browned. Turn over and cook for another 5 mins to brown the other side.
Move the coals to the sides of the barbecue to reduce the heat under the meat and cook more gently for 30-40 mins, turning occasionally. This timing will give you pink meat.
Remove the meat to a large board and cover tightly with foil. Leave to rest for 10-15 mins.
For the sauce, fold the harissa, a little salt and the coriander into the yogurt. Cut the lamb into thick slices and serve with the sauce and couscous, if you like.