"Sauce of Lyon." Demi-glace sauce combined with onions sweated in butter and vinegar. It is usually served with liver or Vienna steak. It is thought that Lyonnaise sauce was invented by Duke Philippe De Mornay (1549-1623), Governor of Saumur, Lord of the Plessis Marly in the 1600s. He was a writer on protestant affairs and known as the protestant pope. It is said that he also invented sauce mornay, sauce chasseur and sauce porto.
