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.