Ham Sauce (Mustard Sauce)

Bring about 3 inches of water to a simmer in the bottom of a double boiler. Whisk together the sugar vinegar tomato soup mustard powder egg yolks and melted butter in the top of the double boiler. Transfer 1/4 cup of the mixture to a small bowl; mix in flour until smooth. Whisk flour mixture back into mustard sauce; combine thoroughly. Cover and turn heat to medium-high.

Cook sauce until thick approximately 30 minutes; stir occasionally. Turn heat to low and simmer sauce to hold.