Preheat the oven to 200˚C, gas mark 6. Heat 1 tbsp oil in a large casserole or ovenproof frying pan. Fry the onion and garlic over a medium heat for 3-4 minutes to soften, then add the harissa paste and fry for another minute. Stir in the tomatoes, then arrange the chicken, skin-side up, on top. Season and transfer to the oven for 35 minutes. Ensure the chicken is cooked thoroughly so the juices run clear and there is no pink meat.
Meanwhile, put the couscous in a mixing bowl with 300ml just-boiled water and cover the bowl. Toss the chickpeas with 1 tbsp oil and the cumin in a roasting tin. Add to the oven for the final 15 minutes of roasting.
To serve, uncover the couscous and fluff up with a fork. Stir in the remaining 1 tbsp oil, lemon zest and juice and herbs. Serve with the chickpeas, chicken and harissa sauce.
For a little extra flavour, add some pitted green olives to the pan with the canned tomatoes and bake with the chicken.