2 tablespoons cold unsalted butter, cut into 4 pieces
1/4 cup whole-grain mustard
Salt and freshly ground pepper
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.
The mustard sauce can be refrigerated overnight. Gently reheat, whisking constantly.