In a large Dutch oven, melt the butter over low heat.
SOFTEN THE VEGETABLES
Add the celery root, the onion, garlic, and thyme to the melted butter. Tumble. Cover and raise the heat to medium-low.
Sweat the vegetables for 25 minutes.
Remove the lid, and add the chicken broth, salt, and the lemon zest.
Bring the soup to a steady simmer, reduce heat, and cook, covered, for at least 20 minutes, until all the vegetables are quite soft.
Remove the lid and carefully pull out the lemon zest with a pair of tongs.
THICKEN AND BLEND A SMOOTH SOUP
Ready a blender or, preferably, an immersion blender.
Add the bread to the soup. Stir with a wooden spoon. It's the 19th century, remember?
In mere seconds, the bread will have broken down into the soup. Now, being extremely careful, blend the soup until silky smooth: Use an immersion blender right in the pot or place about a quarter of the mixture in a blender. Remove the center piece of blender lid (to allow steam to escape); secure blender lid on blender. Drape a clean towel over opening in blender lid (to avoid splatters). Blend in small batches until smooth, pouring into a large bowl.
Repeat procedure until all of the soup is well blended. If you've used a traditional blender, return the soup to the pot.
Stir in the buttermilk and heat the soup over medium-high heat until very hot.
Ladle the hot soup into bowls and garnish with crème fraîche and chives.