With a variation for potato salad dressing.
From the box of L.R. from Winston-Salem, North Carolina.
1/3 cup sugar, mixed with
1 Tablespoon flour
1/2 tsp. salt
1/2 cup (wet) prepared mustard
1/2 cup cider vinegar
1/2 cup hot water (or use 1/4 cup hot water, 1/4 cup sherry)
Cook slowly on low fire (or in double boiler) stirring constantly for 10 minutes (or use beaters to prevent lumping). Beat slowly.
Add: 2 well-beaten eggs, cook slowly, stirring until mixture thickens.
Add: 3 Tablespoons butter, cook slowly 5 minutes, stirring constantly, or beating with beaters, slowly. Cool, and store in refrigerator in mayonnaise jar. Keeps indefinitely. Makes about 1 pint.
As dressing for potato salad, mix equal parts of mustard sauce, sour cream, mayonnaise.