How to Make It
In a small saucepan, combine the onion and white wine and simmer over moderate heat until the wine is reduced to 1 tablespoon, about 5 minutes. Add the heavy cream and bring to a boil. Transfer the mixture to a blender and puree. With the machine on low speed, add the butter, 1 piece at a time, until the sauce is smooth.
Scrape the onion sauce into a small saucepan and stir in the mustard. Season with salt and pepper. Rewarm the sauce over low heat, stirring constantly. Serve the sauce warm.