4 large leeks (white and tender light-green part only)
1 Celery root
6 tablespoons butter, cut into small pieces
8 white potatoes
8 cups chicken or vegetable stock
1 tablespoon salt
1 teaspoon freshly ground white pepper
1 cup heavy cream
Slice leeks in half lengthwise and rinse under cold running water until all sand is gone. Thinly slice leeks against the grain. Using a large, sharp knife, trim the rough ends squarely off the celery root. Place flat-end down on a cutting board and carefully trim off the remaining skin in thin strips from top to bottom. Cut the white flesh into large dice.
Melt the butter in a large, heavy stockpot over low. Add the leeks and celery root and sauté until softened but not browned, stirring often, about 12 to 15 minutes. While the vegetables cook, peel and cut the potatoes into large dice.
Add the potatoes, stock, salt and white pepper to the pot. Bring to a boil, reduce heat and simmer for 20 minutes, until tender. Strain out liquids from solids, reserving both. Working in batches, puree the solids with a few cups of the liquid in a blender or food mill until mostly smooth but still slightly chunky. Return the puree to the stockpot and combine with the liquid. Add the cream and heat through. Adjust seasoning. Serves 8 to 10.