Melt the butter in a large saucepan and gently fry the carrots, leeks and celery for 5 minutes.
Add the flour and cook for 1 minute, then gradually blend in the stock. Add the Bouquet Garni, Mixed Herbs and season to taste. Bring to the boil, then add the potatoes.
Cover and simmer for 25 minutes, or until the soup is to the desired consistency and the vegetables are cooked through.