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1
In a large Dutch oven, melt the butter over low heat.
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2
SOFTEN THE VEGETABLES
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3
Add the celery root, the onion, garlic, and thyme to the melted butter. Tumble. Cover and raise the heat to medium-low.
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4
Sweat the vegetables for 25 minutes.
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5
Remove the lid, and add the chicken broth, salt, and the lemon zest.
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6
Bring the soup to a steady simmer, reduce heat, and cook, covered, for at least 20 minutes, until all the vegetables are quite soft.
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7
Remove the lid and carefully pull out the lemon zest with a pair of tongs.
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8
THICKEN AND BLEND A SMOOTH SOUP
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9
Ready a blender or, preferably, an immersion blender.
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10
Add the bread to the soup. Stir with a wooden spoon. It's the 19th century, remember?
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11
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.
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12
Repeat procedure until all of the soup is well blended. If you've used a traditional blender, return the soup to the pot.
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13
Stir in the buttermilk and heat the soup over medium-high heat until very hot.
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14
Ladle the hot soup into bowls and garnish with creme fraiche and chives.
