Combine vinegar and mustard in a small non-aluminum saucepan; mix well. Cover and refrigerate overnight.
Add beaten egg, sugar, and salt to reserved mixture; stir. Bring to a boil. Reduce heat; simmer 10 minutes or until slightly thickened, stirring well.
Quickly ladle mustard into hot sterilized jars. Cover with a metal lid, and screw band tight. Refrigerate 24 hours before serving. Mustard may be stored in the refrigerator up to 2 weeks. Serve with meats.