Tip the couscous into an ovenproof dish. Stir the salt into the measurement water and pour over the couscous. Stir once to make sure all the grains are submerged in the water, then cover with a clean tea towel and leave to stand for 10–15 minutes. Rake a fork through the couscous to loosen the grains. Then, using your fingers, rub the oil into the grains until light, airy and any lumps are broken up. Scatter over the butter and cover with a piece of damp greaseproof paper. Place in a preheated oven, 180°C (350°F), Gas Mark 4, for 15–20 minutes.
Meanwhile, bring the stock to the boil in a heavy-based saucepan. Stir in the garlic, rosemary, bay leaves, peppercorns and 1 teaspoon salt, then add onions, carrots and sweet potato, reduce the heat and cook gently for 4–5 minutes. Add the courgette, celery and leeks and continue to cook gently for 10 minutes, then stir in the tomatoes, honey and harissa paste and cook gently for a further 5 minutes. Season to taste.
Pile the couscous in a mound on a shallow serving dish. Using a slotted spoon, lift the vegetables out of the stock and place them around the couscous. Pour the stock into individual bowls and serve with the vegetables and couscous.