Put the yogurt and 2 tbsp harissa in a large bowl. Season, add the chicken supremes and turn to coat. Marinate in the fridge for 1 hour.
Preheat the oven to 200°C, gas mark 6. Oil a large roasting tray and arrange the chicken in the tray. Roast for 15 minutes.
Meanwhile, put the pattypans and pepper in a bowl. Add 1 tbsp olive oil and the remaining harissa. Stir to coat then transfer to the tray with the chicken. Roast for 30 minutes more, turning halfway, until the chicken is cooked through and the vegetables are tender.
Coarsely grate the cauliflower into a bowl. Stir in the chopped coriander and remaining olive oil then season.
Divide the vegetables, cauliflower couscous and two chicken supremes between plates. (The remaining chicken can be stored in the fridge for up to two days, ready to be used for the shawarmas and tagine.)