Place the hard-cooked eggs into a 1 quart jar. In a saucepan, stir together the mustard, cornstarch, sugar, turmeric, and salt. Pour in just enough of the cider vinegar to make a paste, then gradually stir in the rest. Bring the mixture to a boil, stirring frequently. Pour into the jars with the eggs. Put the lid on the jar, and refrigerate for a few days before eating for best flavor.