Cindy Pawlcyn serves this dipping sauce with her Mongolian Pork Chops at her Mustards Grill restaurant in Yountville, California.
Place sugar and mustard in the top of a double boiler, and whisk to combine. When well combined, whisk in egg yolks and vinegar. Cook over simmering water, stirring occasionally, until thick enough to form ribbons when drizzled from a spoon, 10 to 15 minutes. Remove from heat, and allow the mixture to cool. When cool, fold in the creme fraiche. Refrigerate in an airtight container until ready to use.