This was a tradition at our house for Easter, along with Pennsylvania Dutch Potato Salad, Gussie’s Spinach Casserole, and Smoked Turkey. I think the sauce was originally intended for ham, but you will likely find all kinds of ways to use it.
1/3 cup dry mustard
1/3 cup cider vinegar
1 egg
1 cup sugar
Mix mustard and vinegar together and let stand 24 hours. Put in top of double boiler on low heat. Beat in egg and sugar. When it begins to thicken, take off heat and continue to beat until thick.