Preheat the oven to 220°C/425°F/gas 7. Click off and discard only the tatty outer leaves from the cauliflower, then cut it into quarters. Quarter the peppers and halve the tomatoes, removing and discarding the seeds. Break the unpeeled garlic bulb into cloves. Mix it all in a large sturdy roasting tray with 2 tablespoons each of olive oil and red wine vinegar, the harissa, a pinch of sea salt and black pepper and a good splash of water. Cover the tray tightly with tin foil, put it on a medium heat on the hob, and when it starts to sizzle, transfer it to the oven for 40 minutes.
Get the tray out of the oven, remove the foil, tilt the tray and spoon all the delicious juices into a little jug for later. Return the tray to the oven for 20 minutes, to get golden. Place the couscous in a bowl, add a pinch of salt and pepper, then just cover with boiling kettle water, and cover. Pick most of the mint leaves into a blender, blitz with 6 tablespoons of yoghurt until smooth, then ripple it back through the remaining yoghurt. Fluff up the couscous. Warm the reserved juices. Squeeze the soft garlic out of the skin. Serve it all together, sprinkled with mint leaves.