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 break up 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 20 minutes until heated through.
Meanwhile, heat the olive oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion and garlic and cook for 2–3 minutes. Stir in the harissa, tomatoes and sugar, then cover and cook over a medium heat for 10–15 minutes. Toss in most of the herbs and season with salt and pepper.
Pile the couscous in a conical mound on a shallow serving dish and use a spoon to create a hollow in the top. Spoon the spicy tomato sauce into the hollow and garnish with the rest of the herbs and the preserved lemon rind. Serve with tagines or grilled meat, poultry or fish.