Melt butter in a large Dutch oven over low heat.

Add diced celery root, sliced sweet onion, garlic cloves, and thyme leaves to the melted butter.

Tumble the vegetables to coat them in butter.

Cover the Dutch oven and raise the heat to medium-low.

Sweat the vegetables for 25 minutes, allowing them to soften without browning.

Remove the lid and add chicken broth, kosher salt, and lemon zest to the pot.

Bring the soup to a simmer, then reduce the heat to low.

Cover the pot and cook for at least 20 minutes, or until all the vegetables are very soft.

Remove the lid and carefully remove the lemon zest using tongs.

Add cubed French bread to the soup.

Stir with a wooden spoon until the bread breaks down and thickens the soup.

Using an immersion blender, carefully blend the soup until it's silky smooth. If using a traditional blender, blend in batches.

Return the blended soup to the pot if using a traditional blender.

Stir in buttermilk and heat the soup over medium-high heat until very hot, but not boiling.

Ladle the hot soup into bowls.

Garnish each bowl with a dollop of creme fraiche and sliced fresh chives.