The Collaborative International Dictionary of English v.0.48:

Soubise \Sou`bise"\ (s[=oo]`b[=e]z"), n.
   1. [F.] A sauce made of white onions and melted butter mixed
      with velout['e] sauce.
      [Webster 1913 Suppl.]

   2. A kind of cravat worn by men in the late 18th century.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Soubise
    n 1: veloute sauce with sauteed chopped onions and whipping
         cream [syn: Soubise, white onion sauce]