Combine the above and allow to sit in the fridge for one day before adding your cultured ingredient. (or, use a store bought mustard – it works just as well).
Finley chop or blend up 1/2 cup of living, raw sauerkraut or cortido. Blending will keep the final mustard consistency smooth. (Alternatively, you can use 2 tablespoons of whey separated from a yogurt with living cultures or brine from a batch of kraut).
Add the mustard base and combine well with the blended or chopped ferment.
Allow to sit on the counter for 1-2 days if temps are over 70ºF or 2-3 days if temps are below 70ºF.