In a small bowl mix together dry mustard and vinegar. Let sit for at least 2 hours. This is crucial for developing the "heat" in the mustard.
After a couple hours combine mustard mixture, egg, and sugar in a small saucepan over low heat. Cook, stirring constantly, until mustard is thickened, about 5-10 minutes. Set aside to cool. If using later, store in airtight container in refrigerator.
Preheat oven to 375ºF.
Lightly beat eggs in a large bowl and then combine with the rest of the ingredients until well mixed.
Roll into 1 inch balls and place on ungreased baking sheet. Bake for 15 minutes until golden and hot. If frozen, add about 10 minutes to time.
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Make Ahead Instructions: Mustard can be made ahead several days ahead of time and stored in refrigerator (I store it in a jar).Spinach balls can be rolled into balls and frozen UNBAKED on cookie sheet. After they are frozen, transfer them to a freezer bag until ready to bake. When ready to bake, place FROZEN spinach balls on cookie sheet, baking 10 minutes longer than instructions.