Step 1

Cut the mushrooms into pieces, then pulse in a food processor with the koji rice to make a coarse meal. Finally pulse in the salt.

Step 2

Transfer the mixture into a non-reactive container such as a glass jar, add the water, mix very well, then press a layer of plastic wrap over the surface and leave out in a cool dry place for at least 30 days. Make sure there's a couple inches between the mushroom mixture and the lid to account for rising during fermentation.

Step 3

During the fermentation process, stir as often as you can remember with a clean utensil, replacing the plastic wrap and wiping down any exposed sides or glass with a paper towel doused with vinegar if you start to see mold. If you make this in the spring when it's cool, it will be easier to control mold.

Step 4

At first, the mixture will darken on top and oxidize—stir this back in—it’s normal and won’t affect the flavor. As the beneficial bacteria take hold they’ll stem any discoloration, but it will take some time for them to colonize.

Step 5

Mold is bad here, and should be removed quickly as it can give off-flavors. That being said, the high amount of salt and vigorous fermentation from the koji give a solid foundation that, once colonized (and it colonizes quickly) should be bulletproof to nasty bacteria. Basically I’m saying this is not a risky ferment—it’s very easy, much safer than aging something like salami, say.

Step 6

After the mixture has fermented for 30 days, strain it twice, then, for the clearest result, allow it to drain through a coffee filter. After straining, bottle and store in the fridge. Know that the mixture is still alive, and fermenting, and you will want to burp the jar here and there to release carbon dioxide.