1 stick (4 ounces) plus 1 tablespoon unsalted butter
3 large leeks, white and tender green parts only, halved lengthwise and
thinly sliced crosswise
2 thyme sprigs, plus 1 teaspoon minced thyme
1/2 cup dry white wine
6 cups chicken stock or canned low-sodium broth
2 1/2 pounds celery root—quartered, peeled and cut into 2-inch chunks
1/2 pound Yukon Gold potatoes, peeled and cut into 2-inch chunks
1/2 cup heavy cream
1 tablespoon minced shallots
1/2 pound medium shrimp, shelled and deveined, shells reserved, shrimp halved
Freshly ground pepper
1/4 cup finely diced Granny Smith or other tart green apple
Melt 3 tablespoons of the butter in a large enameled cast-iron casserole. Add the leeks and thyme sprigs and cook over low heat until softened, about 8 minutes. Add the wine and simmer over moderately high heat for 2 minutes. Add the chicken stock, celery root, potatoes and a large pinch of salt and bring to a boil. Cover and simmer over low heat until the celery root and potatoes are tender, about 45 minutes.
Working in batches, puree the soup in a blender until very smooth. Return the soup to the casserole and stir in 1/4 cup of the heavy cream.
In a medium skillet, melt the remaining 6 tablespoons of butter. Add the shallots and the reserved shrimp shells and cook over low heat, stirring, for 8 minutes. Strain through a coarse strainer, pressing on the shells to extract as much butter as possible; you should have about 2 tablespoons. Keep warm.
In a small saucepan, bring the remaining 1/4 cup of heavy cream to a simmer. Add the shrimp and a pinch of salt and cook over low heat, stirring, until just cooked through, about 1 minute. Remove from the heat.
Reheat the soup and season with salt and pepper. Ladle the soup into warm shallow bowls. Spoon the shrimp and cream into the bowls. Drizzle each serving lightly with the shrimp butter, garnish with the diced apple and minced thyme and serve at once.
The recipe can be prepared through Step 3 up to 2 days ahead and refrigerated. Melt the shrimp butter over low heat before using.