Pour the boiling water over the couscous, stir, then leave until absorbed. Heat the coriander and cumin seeds in a small pan until fragrant. Grind to a powder, then mix with the cinnamon. Heat 1 tablespoon of the oil in a frying pan, add the pine nuts and almonds and fry until browned. Transfer to kitchen paper to drain. Add the remaining oil to the pan. When hot, add the onion and fry until soft. Stir in the garlic and spice mixture and fry for 2 minutes, then add the nuts, mint, coriander, raisins, couscous and salt and pepper.
Open out the lamb, skin side down, on a work surface. Season inside with pepper, then spread over the stuffing. If possible, tuck the flaps of the piece of lamb over the stuffing. Roll up the lamb into a neat sausage shape, then tie it securely with string.
Put the onion wedges into a roasting tin in which the lamb will just fit. Place the lamb on top of the onion wedges and pour over the oil and lemon juice. Cook in a preheated oven, 240°C (475°F), Gas Mark 9, for 15 minutes, then reduce the temperature to 220°C (425°F), Gas Mark 7, and cook for a further 25 minutes, so that the lamb is pink in the centre. Remove the lamb from the oven, cover and leave to stand in a warm place for about 15 minutes before carving.