Combine all ingredients, except eggs, in top of a double boiler; cover and let stand at room temperature 4 to 6 hours.
Add beaten egg to reserved mixture, stirring well with a wire whisk. Cook over simmering water, stirring constantly, 8 minutes or until thickened.
Quickly ladle mustard into hot sterilized jars. Cover with metal lids, and screw bands tight. Refrigerate 24 hours before serving. Mustard may be stored in refrigerator up to 3 months.
Oxmoor House Homestyle Recipes