In a large cast-iron fry pan, roast a cup or so of whole wheat pastry flour, stirring frequently with a wooden spoon. The aroma is wonderful.
Add the toasted flour to a bowl with an equivalent amount of ground yellow mustard seed. (More flour for a milder mustard; less for hotter.)
Add other dry ingredients you like--my favorites are dried crumbled tarragon and garlic salt. I plan to try some turmeric one of these days.
Stir in vinegar (balsamic is special), water, and honey until you get it to the right consistency and taste. Put in those cute little jars you've saved.
It tastes smooth with the bit of sweetness from the honey, and then suddenly you're hit from behind with an intensity that makes your eyes tear. That's good mustard.