1.To make harissa, place chilli in small heatproof bowl of boiling water; stand 1 hour. Drain; reserve ¼ cup soaking liquid. Dry-fry spices in small frying pan until fragrant. Blend or process spices, chilli, reserved soaking liquid, garlic and paste until mixture is smooth; transfer to small bowl, stir in rind and juice.
2.Preheat oven to 180°C/350°F.
3.Rinse chicken under cold water; pat dry inside and out with absorbent paper. Tuck wing tips under chicken. Fill large cavity with orange. Make a pocket between breast and skin with fingers; rub 2 tablespoons of the harissa under skin inside pocket. Tie legs together with kitchen string; brush chicken all over with 2 tablespoons of the harissa.
4.Half-fill large shallow baking dish with water; place chicken on oiled wire rack over dish, season. Roast, uncovered, about 1 hour.
5.Meanwhile, heat oil in large flameproof baking dish; cook onions, potatoes and unpeeled garlic, stirring, until vegetables are browned.
6.Cover chicken; roast about 50 minutes or until chicken is cooked through. Add eggplant and tomatoes to vegetable mixture in dish, season; place in oven for about the last 20 minutes of chicken cooking time or until vegetables are tender.
7.Serve chicken with roasted vegetables and remaining harissa.