1-2 pounds celery root

1-2 pounds potatoes 

1 cup milk and/or cream

4 oz butter

Salt and pepper

Most recipes for celery root-potato puree call for anywhere from equal parts (by weight) potatoes (russets or gold potatoes like the Weiser satinas) to 2 part potatoes to 1 part celery root. 
Peel potatoes and celery root. Cut potatoes into ½ inch dice and celery root into 1-inch dice. (The celery root cooks faster.) 
Add to pot and cover with cold water and a couple teaspoons of salt. Bring to a boil and then reduce heat and simmer until very tender. Drain and dry. 
Heat 1 cup of milk and cream in any combination (½ cup milk and ½ cup cream or all cream or milk). 
Pass potatoes and celeriac through a rice or food mill or mash them with butter. (The fat coats the starches and prevents them from getting gummy.) 
Once all the vegetable is pureed, stir in the milk/cream. Adjust seasoning.

Delicious with the Mississippi pot roast.