Combine mustard, sugar, cornstarch, turmeric, and seasoned salt in top of a double boiler; stir in 1/2 cup water, blending well. Add remaining water and vinegar; stir well. Cook over simmering water, stirring until thickened. Remove from heat; stir in basil, savory, and tarragon.
Spoon mustard into hot sterilized jars. Cover with metal lids, and screw bands tight. Refrigerate overnight before serving.