Preheat the oven to 180°C/350°F/gas 4. Peel and quarter the onions and place in a large roasting tray. Prick and add the whole chillies. Halve the unpeeled garlic bulb across the middle and add to the tray with the harissa, paprika, a pinch of sea salt and black pepper, 2 tablespoons of red wine vinegar and 1 tablespoon of olive oil. Use a large sharp knife to carefully cut down the back of the chicken, so you can open it out flat. Add to the tray and toss well, rubbing all that flavour into the bird. Place the chicken, breast side up, directly on the top bars of the oven with the tray of onions beneath it. Roast for 1 hour, or until the chicken is golden and cooked through. Scrub the potatoes, chop lengthways into chunky wedges, toss with 1 tablespoon of oil and a pinch of seasoning, then arrange in a single layer on your largest roasting tray and place in the oven below the onions.
After 30 minutes, remove the onion tray from the oven, basting the chicken with any juices from the tray. Shake the wedges and move up to sit under the chicken for the remaining 30 minutes. For the sauce, halve and deseed the chillies, squeeze half the soft garlic flesh out of the skins, then blitz both in a blender with the onions and all the tasty juices from the tray, half the mint leaves and 1 tablespoon of red wine vinegar until smooth, adding a good splash of water, if needed, then season to perfection. Serve the chicken with the wedges and peri peri sauce, scattered with the rest of the mint leaves and roasted garlic.