1 (2 oz.) can dry mustard
1/2 c. sugar
1/2 c. distilled white vinegar
1 egg, beaten

Combine mustard and sugar in top of double boiler. Gradually whisk in vinegar, then egg. Set over simmering water and cook until mixture darkens slightly and mellows in flavor, stirring frequently, about 12 minutes. Transfer to a jar and refrigerate.