1/2 c. dry mustard
1/2 c. vinegar
1 egg
1/4 c. sugar
Dash of salt
1 c. mayonnaise

Mix mustard and vinegar thoroughly. Cover and let stand overnight. Next day, beat egg and stir in sugar and salt. Add mustard mixture. Cook until mixture thickens slightly and coats spoon. Cool. Blend mayonnaise into cooled mixture. Keep refrigerated. Will keep for several months.