Preheat oven to 200°C/180°C fan forced. Tie drumsticks together with kitchen string. Place chicken in a large baking dish. Combine oil, harissa paste and garlic in a bowl. Brush harissa mixture over the chicken. Cover loosely with foil and roast for 1 hour and 30 minutes.
Remove foil. Bake chicken for a further 45 minutes, brushing occasionally with remaining harissa mixture, or until golden brown and cooked through (juices run clear when tested with a skewer). Remove chicken from oven and rest for 5 minutes.
Whisk yoghurt, onion, tahini paste, lemon juice, the water and coriander in a bowl. Season with salt.
Serve chicken with tahini yoghurt and lemon wedges.