French grandmothers have built reputations off of soups such as this one, using whatever leftovers lurked in the vegetable bin, simmered in good broth, then pureed. Pale button mushrooms yield a prettier soup than the dark-skinned crimini mushrooms. To splurge, top the soup with a few drops of truffle oil.
Melt the butter in a large pot over moderate heat. Add
the leeks and sauté until softened, about 4 minutes. Add
the celery root, mushrooms, potatoes, thyme, and broth and
bring to a simmer. Cover and adjust the heat to maintain a
gentle simmer. Cook until the vegetables are tender, about
20 minutes.
Remove the thyme sprigs. Let cool slightly, then puree
the soup in a blender or food processor until smooth,
in batches if necessary. Return the soup to a clean pot and
reheat to serve, thinning to the desired consistency with
additional broth. Season with salt and pepper.
Divide among warmed soup bowls, garnishing each
portion with a drizzle of crème fraîche and a sprinkle
of parsley.
