A Russian fermented tonic of either rye bread or vegetables, kvass, can be used to make cold nutritious soups. Click for the pravda about kvass beet borscht, kvass mixed vegetable soup, and a cucumber melon soup enriched with kvass, da?
Instead of soaking the mustard seeds in the traditional wine, beer or vinegar, choosing whey or pickle brine (such as from making dill pickles) introduces the health benefit and keeping qualities of lactic acid bacteria into your condiment.
When you go to all the effort to make a condiment from scratch, you want it to have some legs. It should last at least a few weeks; even better a couple of months. Let me introduce you to the art of culturing.