In a saucepan, over medium heat, melt the butter. Add the onions and saute for 1 minute. Add the wine and bring to a boil and cook for 2 minutes. Stir in the mustard. Add the demi-glace and reduce the heat to medium. Simmer for 20 minutes. Remove from the heat and serve. The sauce is great on grilled meats, especially on pork.
Recipe courtesy of Emeril Lagasse, 1998