Heat the oil in a large saucepan over a moderate heat. Sauté the onions and capsicum for 5 minutes. Add the carrots, zucchini and celeriac, then sprinkle with the spices and cook for a few seconds.
Pour in the stock. Bring to a gentle boil, then add the tomato paste and bouquet garni. Reduce the heat, then cover and cook gently for 25 minutes. Add the chickpeas to the vegetables and cook for a further 10 minutes.
Meanwhile, put the couscous in a large heatproof bowl and pour over 2 cups of boiling water. Stir in a pinch of salt, then cover and leave to swell for 5 minutes, or until the couscous has absorbed all the water.
Add the butter to the warm couscous, fork it through, then tip into a warmed deep dish. Using a slotted spoon, pile the vegetables on top of the couscous, removing the bouquet garni. Pour over some of the vegetable cooking juices, then serve the remainder in a jug. Serve piping hot, garnished with sprigs of fresh mint.