Peel the carrots and cut into matchstick-thin slices. Peel and shred the onions. Wash and shred the leeks and the celery. Melt the butter in a large saucepan, add the prepared vegetables and sauté gently to soften but not brown. Add the hot chicken stock and the bouquet garni. Cover with a lid and simmer gently until the vegetables are tender.
Remove the bouquet garni and taste the brot