1/3 to 1/2 c. dry mustard
1/2 c. white vinegar
1/2 c. sugar
1 egg yolk

Combine dry mustard and vinegar in small bowl. Cover and let stand at room temperature overnight. Heat with sugar over low heat. Whisk 1 tablespoon of mustard into the beaten egg yolk. Add egg mixture to saucepan and simmer over very low heat 1-2 minutes until slightly thickened. Cover and store in refrigerator. Serve at room temperature.