In a nonmetallic bowl, preferably china or glazed pottery, blend together the dry mustard and sugar.
Whisk in both vinegars and the salt. Let this stand, uncovered, for 24 hours.
To finish the mustard, transfer the mustard solution to a nonmetallic saucepan (I use enameled cast iron). Whisk in the beaten eggs. Cook the mixture over moderately low heat, stirring continuously, for about 17 minutes, or until dense enough to thickly coat a spoon.
Pour the cooked mustard into a jar, cool to room temperature and then cover and refrigerate.