Prep Time: 1 hour
Source: My Paris Kitchen by David Lebovitz
The recipe calls for crème fraîche because its sturdy richness stands up well to to the soup, but you can use heavy cream instead. If you do use heavy cream, whip it very stiff, to the point where it just about turns to butter.
1 1/2 cups crème fraîche
Generous pinch of sea or Kosher salt
2 teaspoons fresh squeezed lemon juice
2 Tablespoons prepared horseradish
6 slices prosciutto
4 large leeks, cleaned
6 Tablespoons butter, salted or unsalted
2 1/2 teaspoons sea salt or Kosher salt, plus additional if needed
1 1/2 pounds celery root
1 bay leaf
6 sprigs thyme
6 cups water
1/2 teaspoon fresh ground white pepper
Minced fresh chives, for garnish
Prepare the horseradish cream. Whip the crème fraîche in a metal bowl with a whisk until it thickens. It should hold it’s shape when you remove the whisk from the bowl. Whisk in the salt and lemon juice. Stir in the horseradish. Let the mixture chill for an hour to allow the flavors to meld. (It is best make 3 to 4 hours in advance.)
Prepare the ham chips. Preheat the oven: 350° F. Line a baking sheet with parchment paper. Lay the slices of prosciutto on the paper, evenly spaced. Bake for 5 minutes. Turn the slices midway through. Watch the ham carefully as the thickness determines how quickly they crisp up. When they feel dry and firm, remove from the oven and let them cool until crisp. Store the chips in an airtight container until ready to use.
Prepare the soup. Slice the leeks into 1/4-inch slices, white and light green parts, only. Melt the butter in a large Dutch oven over medium heat. Add the sliced leeks and cook for 10 minutes until the leeks are completely soft. Add salt midway through cooking.
As the leeks are cooking, peel the celery root. To peel the celery root, cut off the top and bottom ends of the root and set it on a cutting board, flat side down. Cut away the coarse skin, slicking it with a curved motion to match the curvature of the root. Conserve as much flesh as possible. Slice the root into 3/4-inch thick slices. Cut the slices into cubes. Add the cubes to the pot along with the water, bay leaf, and thyme. Bring to a boil. Decrease the heat so that the mixture is at a simmer. Cover with the lid askew. Cook the mixture for 30 to 40 minutes until the celery root is tender. Pierce some cubes to check for tenderness.
Remove the bay leaf and the thyme. Let the soup cool to tepid. Add the white pepper. Blend the soup until completely smooth. A hand blender or processor may also be used. If using a blender and the soup is still warm, only fill the container halfway. At this point the soup may be stored covered, in the refrigerator for up to 4 days.
To serve the soup, reheat and ladle into serving bowls. Add a generous dollop of horseradish cream and then crumble the ham chips over the top. Sprinkle chives over the top and serve.