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Remove the lamb from the fridge and allow to come to room temperature while you prepare the stuffing. Heat half the oil in a large frying pan, add the onion and fry for 8-10 mins until starting to turn golden. Add the garlic and spices and cook for 1 min more, then scoop out into a large bowl. Add the breadcrumbs, herbs, pistachios, feta, cherries, lemon and tahini. Season and mix everything together. Unroll the lamb and season well on both sides. Pack the stuffing down the centre of the lamb, pressing to help it stick, then re-roll and tie up with kitchen string (see tip, below left). Flip the lamb over so it is seam-side down, and pack any stuffing that has fallen out back in. Heat oven to 200C/180C fan/gas 6.
Now assemble the gratin. Slice the squash in half to separate the thinner top half and the bulbous bottom half. Peel both pieces with either a sturdy peeler or a sharp knife. Cut down the centre of the thinner piece, then slice very thinly into half moons. Halve the bottom piece and scoop out the seeds, then thinly slice this too. Whisk the tahini, cream and stock in a jug with some seasoning. Arrange the sliced squash in a shallow ovenproof baking dish, approximately 22 x 30cm, seasoning well between the layers. Finish with a layer of neatly lined-up half moon slices. Pour over the tahini stock.
Place the lamb on top of the gratin, seam-side down, and rub with the remaining oil. Place in the centre of the oven and roast for 1 hr 20 mins for slightly pink lamb. Lift the lamb off the gratin and transfer to a plate, covered with foil to keep warm, and leave to rest. 4 While the lamb rests, increase oven to 220C/200C fan/gas 7. Spoon any excess fat off the top of the gratin, then return to the oven for 20 mins until golden and crisp. Place the lamb back on top of the gratin to serve, scattered with mint leaves and a pot of Saffron yogurt (see tip, below left) on the side.
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