Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the garlic, onion and coriander and cumin seeds and cook for 1–2 minutes to allow the flavours to mingle. Add the chicken thighs and lightly brown on each side.
Pour in the lemon juice and enough water to just cover the chicken. Stir in the saffron, cinnamon sticks and butter and bring to the boil, then reduce the heat, cover and simmer for 15 minutes. Add the preserved lemon rind and olives, re-cover and simmer for a further 10 minutes. Season to taste and serve hot with the couscous, if liked.