Heat oven to 200C/180C fan/ gas 6. Mix 2 tbsp of the harissa with 3 tbsp of the yogurt. Rub all over the chicken breasts and set aside to marinate while you start the veg.
Toss the squash and the onions with remaining harissa, mixed with 2 tbsp oil (sunflower, vegetable or olive is fine), and some seasoning in a large roasting tin. Roast for 10 mins.
Remove veg from the oven, add the chicken to the tin, then roast for a further 20-25 mins until the chicken and veg are cooked through. Serve with the remaining yogurt on the side, and a big bowl of couscous or rice.