Celery Root Puree w/French Fridays with Dorie

Two celery roots, a potato, an onion, all simmered in milk, then drained and pureed with butter and salt; the perfect complement to short ribs or even a rotisserie chicken.  They don't look like much in the store, but trust Dorie.  Celery roots are ever more interesting than straight mashed potatoes.  Try it!  

Recipe for Go-With-Everything Celery Root Puree, p. 354 from Around My French Table, by Dorie Greenspan