This unusual pairing yields a flavor-rich, smooth soup that can be served steaming or chilled (top with a little sour cream). Like many soups, this one develops nuance if allowed to rest overnight.
Heat oil in heavy large pot. Stir in onion, cover, and cook over moderate heat 5 minutes. Uncover and brown lightly, about 5 minutes.
Meanwhile, peel and chunk sweetpotatoes. (Trim off celeriac tops, if present, and reserve.) Scrub and casually peel root (bits of skin are no problem in puree). Cut into fairly thick slices.