2 tablespoons (1/4 stick) unsalted butter
1 medium onion, coarsely chopped
1 large shallot, coarsely chopped
2 very large leeks, carefully washed and cut into 1-inch rings (with some green)
1 rib celery, coarsely chopped
2 medium baking potatoes, peeled and cut into medium cubes (about 3 cups)
1 small celery root, peeled and cut into medium to small cubes (about 1 ½ cups)
1 teaspoon salt
½ teaspoon white pepper
4 cups of Chicken Stock, heated
Melt butter in a deep saucepan and sauté onion and shallot over medium heat until onions begin to turn brown, about 5 minutes. Add leeks and celery. Cover tightly and sweat for 15 minutes over very low heat.
Add potatoes, celery root, salt, pepper and 1 ½ cups of the stock to the saucepan. Bring to a boil and simmer, uncovered, for 10 minutes or more, until potatoes and celery root are fork-tender.
Transfer soup mixture to a food processor and puree until smooth. Return to the saucepan and add the balance of the stock. Simmer for a few minutes, then correct seasoning if necessary. Serve medium hot.