2 Tbs. unsalted butter
6 cups thinly sliced celery (reserve 1/4 cup celery leaves)
1 medium yellow onion, chopped
Kosher salt
3 cups washed, chopped leeks
6 cups lower-salt chicken broth
3 sprigs fresh thyme
1 fresh or 1/2 dried bay leaf
2 Tbs. whipping cream
Freshly ground black pepper

In a 4-quart pot, melt the butter over medium heat. Add the celery, leek, onion, and a generous pinch of salt. Cook until tender and just beginning to color, 6 to 8 minutes. Add the celery root and stir to coat with the butter. Pour in the chicken broth, and then add the thyme, bay leaf, and 1/2 tsp. salt. Bring to a boil; then reduce the heat and simmer until the vegetables are very tender, about 15 minutes. Discard the thyme sprigs and bay leaf and purée the soup in a blender until smooth. Pass the soup through a medium-fine sieve and transfer to a clean pot. Bring the soup back up to a simmer, whisk in the whipping cream, and season to taste with salt and pepper. Keep hot.

Ladle the soup into bowls and top with celery leaves.