1/2 c. dry mustard
1/2 c. cider vinegar
1 egg, beaten
1/2 c. sugar
1 tsp. cornstarch
1 c. mayonnaise

Mix mustard and vinegar together and let sit overnight in the refrigerator. In saucepan, add egg, sugar, salt, and cornstarch. Cook slowly, stirring until it boils and thickens, 10-15 minutes. Let cool, then stir in mayonnaise and refrigerate.