Place about 5 cups of water in a large pot fitted with a steamer and a lid.
Bring to a boil.
Chop off the roots and peel off the outside layer of skin from the celery roots, being careful to remove all the brown.
Cut each celery root into about 12 pieces, and place pieces in the steamer.
Steam until very soft when poked with a fork, about 20 minutes.
Transfer the celery root to a food processor.
Add the cream and butter and puree until smooth. (If you don't have a food processor, you can use an electric mixer.)
Add additional cream or butter to achieve desired consistency, or, for a healthier option, omit the butter and cream and use a little olive oil instead.
Sprinkle with salt and pepper.