Heat a large non-stick casserole dish or pan on the hob. Add the harissa, spices and onions, stir and cook gently for 10 mins until soft. Add chicken and brown for 5-10 mins. Add squash, stirring to combine, and a splash of water if it starts to stick. Cook for 5 mins more.
Tip canned tomatoes into the pan with ½ can of water, cover and simmer for 20-30 mins. Add the lemon zest and juice, cherry tomatoes, couscous and seasoning. Cover and turn off the heat. Leave on the hob for 10 mins, then stir through the coriander and serve.