Celery and celery root combine to make this a double celery soup. Just a small amount of cream is needed to make it extra creamy. If you've got a bunch of celery with their leaves, you can use the leaves to garnish the soup. — The Beekman 1802 Heirloom Vegetable Cookbook
Also known as celeriac, this rough globular swollen root is often muddied, gnarled, and full of wild offshoot stubbles. Underneath the root's burly exterior, though, is a bright white flesh with a crisp, apple-like texture. The firm flesh has a mild herbaceous quality with celery-like undertones.
Reprinted from The Beekman 1802 Heirloom Vegetable Cookbook by Brent Ridge and Josh Kilmer-Purcell. Copyright (c) 2014 by Beekman 1802, LLC. By permission of Rodale Books. Available wherever books are sold.
In a medium saucepan, melt the butter over medium-low heat. Add the celery, celery root, onions, and potatoes and sprinkle with the salt. Cover and cook for 25 minutes, stirring occasionally, until the vegetables are tender but not browned.
Add the broth and thyme and bring to a simmer. Cook for 15 minutes, or until the vegetables are almost falling apart.
Working in batches, transfer to a blender, add the cream, and purée until very smooth. Season to taste with more salt, if you like, and serve hot.