In a medium saucepan, stir together sugar, flour, and mustard. Mix well. Stir in vinegar and mix thoroughly. Heat over medium heat and stir in milk. Continue to heat, stirring frequently, until mixture comes to a boil. Boil for 3 minutes, until mixture thickens. Watch carefully, as it can burn easily. Remove from heat. Serve hot in a small pitcher. May be stored in an airtight container, refrigerated, for 1 week.
Source: Hannaford fresh Magazine, November - December 2010