Sweat 1 medium onion (6 ounces/150 grams), chopped, in ½ ounce (15 grams) butter until translucent. Add 7 fluid ounces (200 milliliters) white wine and reduce by two-thirds. Moisten the onion–white wine infusion with 1¼ cups (300 milliliters) demi-glace. Cook for 10 minutes to infuse the flavor of the onions thoroughly. The sauce may either be strained or the onions may be retained. Finish the sauce with a pinch of sugar (to bring out the flavor of the onions) and 1 tablespoon (15 milliliters) Dijon mustard. Do not allow the sauce to come to a boil after adding the mustard.