1 Preheat the oven to 140˚C/120˚C fan/gas mark 1. Using a sharp knife, score the lamb in a grid pattern (forming roughly 2cm squares), cutting through the fat, but not into the meat.
2 Place a small saucepan or frying pan on a high heat and add the coriander and cumin seeds.
3 Cook the seeds, tossing every now and then, for 1-2 minutes or until they have darkened slightly in colour. Tip into a mortar with the salt and peppercorns and crush with a pestle until slightly coarse, or place in a plastic bag and crush with a rolling pin instead.
4 Place the lamb in a roasting tin and rub the softened butter all over it, then the garlic. Scatter the spice mix all over the lamb, pressing it in to help it stick to the butter.
5 Place the lamb in the oven and cook for about four hours or until the meat is meltingly tender and golden brown on the outside.
6 Just before the lamb is cooked (though the cooked lamb will sit somewhere warm for about an hour), prepare the yoghurt with cucumber and mint. Place all the ingredients in a bowl and mix together, seasoning with salt and pepper to taste.
7 Carve the lamb into chunks and serve with the yoghurt and cucumber and couscous.