Source : Webster's Revised Unabridged Dictionary (1913)

Lyonnaise \Ly`on`naise"\, a. [F. lyonnaise, fem. of lyonnais of
   Lyons.] (Cookery)
   Applied to boiled potatoes cut into small pieces and heated
   in oil or butter. They are usually flavored with onion and
   parsley.

Source : WordNet®

lyonnaise
     adj : cooked with onions