Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Remove the seeds from the peppers, wash and cut into large pieces. Peel and quarter onions and separate the layers. Mix everything in an ovenproof dish (40 × 30 cm) with sea salt and pepper.
Wash the chicken, pat dry. Use a sharp knife to cut open the spine so that it can be spread out flat, cut the legs in. Mix the harissa paste with some sea salt, pepper and red wine vinegar. Rub the chicken thoroughly with it and do not leave out a single square centimetre.
Lay the chicken flat on the vegetable bed with the skin on top and roast it in a hot oven for about 50 minutes until it is crispy and cooked through. Garnish with plucked mint leaves before serving.