Secret Mustard Sauce

1 cup 4% milk (whole milk, do not substitute)
2 egg yolks  (do not use whites)
1 Tablespoon plain flour
1 Tablespoon dry Coleman's mustard
3/4 cup sugar
1/2 cup White House apple cider vinegar

Beat egg yolks and milk with whisk.

Add dry ingredients to the egg/milk liquids in a double boiler,
and cook over hot water until thickened. 

Whisk mixture constantly, about 10-15 minutes.  Sauce will
thicken slightly.

Gradually (slowly) whisk in the vinegar and continue stirring
until heated throughly.  Sauce will thicken as it cools,
serve warm with ham, or cold as a chef salad dressing.

Yield: 1 1/2 cups of sauce