Preheat oven to 180°C. Combine the ginger, cumin, ground coriander, olive oil and 1 tsp each salt and pepper.
Rub over the chicken, onion, sweet potato and garlic, then place all in a lined roasting tray. Place in the oven and cook for 40 minutes or until golden and cooked through.
For the couscous, combine all the ingredients in a bowl and keep warm.
Combine the yoghurt and harissa. Set aside. Remove chicken from the oven and serve with couscous, yoghurt and top with fresh coriander.