Actually, it is really easy. Freeze freshly bought natto in the freezer (It will keep this way for about a month).
When you want to eat it Remove the mustard from the styrofoam tray, and heat the natto in the microwave for 40 seconds (500 W). Or defrost it in the fridge overnight.
Now eat it just as you normally would.
Story Behind this Recipe
Natto is often sold in packs of 3 or 4. It is difficult to eat it all by the "best by" date, so I use this method to keep it fresh. A lot of people don't know this method, so I thought I'd share it here.
Chopped natto won't come unstuck quickly after freezing, so microwave for a short time to loosen. It is easier to mince if it is still mostly frozen. I tried a pack that was stored for 4 months, but the texture wasn't good and the grains were dehydrated. I recommend keeping the natto for up to a month.