The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lyonnaise
    adj 1: cooked with onions