Preheat the oven to 200 degrees C. Arrange all the vegetables in a large shallow roasting tin, leaving the garlic cloves whole but peeled. Drizzle over the olive oil and season well with black pepper.
Roast the vegetables for 45–50 minutes, turning them occasionally, until tender.
Meanwhile, put the couscous in a deep ovenproof dish, pour over 500ml boiling water, cover and leave for 5 minutes to allow the grains to soak up the water. Cover tightly with foil and put in the oven with the vegetables for their final 15 minutes, to keep warm.
Fluff up the cooked couscous with a fork, stir in the butter and herbs and season to taste. Serve the roasted vegetables piled on top of the couscous. Sprinkle with harissa or chilli sauce, to taste.