The soup takes about an hour to prepare -- mainly because the vegetables need to cook for about 30 minutes to become soft enough to puree. When serving the soup, pair it with a frisee salad.
In a medium stockpot, heat butter and oil over medium heat. Add onion and shallot, and season with salt. Cook until soft, about 10 minutes. Add garlic, leek, and celery. Season with salt and cook until vegetables are soft, about 8 minutes. Add celery root and chicken stock, and bring to a simmer. Cover, and cook until celery root is tender, about 30 minutes.
While still hot, puree soup in batches until completely smooth. Season with salt and pepper.
Combine syrup and brandy. Drizzle over hot soup and serve.
This soup freezes beautifully- ladle into a storage container, and reheat for an easy weekday meal (But once you've defrosted the soup, you shouldn't refreeze it.)