Make the chermoula by putting all the ingredients into a food processor with some seasoning. Blend to a smooth paste, then set aside.
Heat the oil in a large, flameproof casserole. Fry the lamb in 2 batches until lightly browned all over, then lift onto a plate. Add the onions to the pan and fry over a medium heat until soft and nicely browned. Add the tomatoes, cinammon, ginger, honey and 2 tablespoons of the chermoula, and simmer for 2-3 minutes.
Return the lamb to the casserole and add the preserved lemon or strips of lemon zest and the hot stock, and season well to taste. Partially cover the tagine and leave to simmer gently for 1 hour.
Gently stir in the squash and chickpeas and simmer, uncovered, for another 15-20 minutes until the squash is tender and the sauce has reduced a little more.
Meanwhile, put the couscous and seasoning into a large bowl and stir in 450ml boiling water. Cover with a tea towel and leave to soak for 5 minutes. Then uncover and fluff up into separate grains with a fork.
Melt the butter in a large pan, then add the couscous and stir over a low heat for a couple of minutes until heated through.
When the lamb and squash are tender, stir in the remaining chermoula and adjust the seasoning if necessary.
Serve the lamb tagine with the buttered couscous, garnished with the extra chopped coriander.
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