Toss the chicken thighs in half the spices and a pinch of salt until completely coated. Heat 1 tbsp oil in a large sauté pan with a lid. Fry chicken, skin-side down, for 10 mins until golden brown, turn over, then cook for 2 mins before removing from the pan. Pour the rest of the oil into the pan, then fry the onions and garlic for 8 mins until golden. Stir in the rest of the spices, then cook for 1 min longer. Pour over the chicken stock and scatter in the olives. Bring everything to the boil, turn down the heat, then sit the chicken, skinside up, in the stock.
Cover the pan with a lid, then simmer gently for 35-40 mins until the chicken is tender. Put the kettle on, then lift the chicken onto a plate and keep warm. Take the pan off the heat. Stir the lemon juice and couscous into the saucy onions in the pan and top up with enough boiling water just to cover the couscous if you need to. Place the lid back on the pan, then leave to stand for 5 mins until the couscous is cooked through. Fluff through half the parsley and the lemon zest, then sit the chicken on top. Scatter with the rest of the parsley and zest before serving.