From Food From the Heartland:

Hot and Sweet Country Mustard

With so many excellent mustards on the market, why make your own? Well, mainly because you can't, as far as I know, buy one like this, and it's so very good. Mustards like this have been made in the Midwest as long as there have been kitchens there.

Combine the dry mustard, flour, sugar and salt, then slowly stir in the cider vinegar. Don't taste it now, because it will be horrible, but in 3 to 4 days it will have mellowed and become superb. Keeps indefinitely in a cool place.

Makes a bit less than 1 cup