How to Make It
Melt the butter in a large saucepan. Add the leeks, celery and caraway seeds and cook over moderate heat until the leeks soften, about 10 minutes. Transfer to a large bowl. Wipe out the saucepan.
Meanwhile, prepare an ice water bath. In a large pot of boiling salted water, blanch the chicory in batches until tender, 1 1/2 to 2 minutes. Using a slotted spoon, transfer the chicory to the ice bath to cool. Drain and squeeze dry, add to the leeks and toss.
Working in batches, in a blender, puree the vegetables and stock. Strain the soup through a coarse sieve into the saucepan; bring to a simmer. Stir in the vinegar and honey. Season with salt and white pepper and serve.