Empty a two-ounce can of Colman’s dry mustard into a large measuring cup. Fill to 1-cup line with cider vinegar. Stir. Let stand an hour.
Beat two eggs in top of a warm double boiler with a dash of salt. Add ½−¾ cup sugar, depending on how sweet you want it. Add mustard mixture and cook over boiling water until thickened, stirring constantly. Store in refrigerator for up to two weeks.