Combine first 8 ingredients in top of a double boiler. Cover and let stand 4 to 6 hours.
Add beaten egg to reserved mixture, stirring well. Cook over simmering water, stirring constantly, 8 minutes or until thickened. Stir in horseradish.
Quickly ladle mustard into hot sterilized jars. Cover with metal lids, and screw bands tight. Refrigerate at least 24 hours before serving. Serve with meats.