½ cup dry mustard
½ cup white vinegar
1/3 cup sugar
1 cup mayonnaise
In a glass bowl or jar mix the dry mustard and the vinegar. Cover and let stand over night.
In a double boiler, beat the egg, stir in the sugar and a dash of salt. Add the mustard mixture.
Cook over hot(not boiling) water until the mixture thickens.
Let cool. Before serving add the mayonnaise.