How to Make It
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.