When 1st hubby was in the AF, we were friends with a couple-the wife was Japanese-once when she and I were sampling each others’ ethnic foods, I gave her some marinated and grilled rattlesnake and she gave me a container of some brownish gooey stuff-fermented soybean sauce-explaining that it is made by cooking the soybeans in saki, putting in a pottery container and allowing it to ferment (she used the words-”you let it rot”) for 2 weeks. I thanked her-it smelled like sour milk left in the sun-I didn’t eat any, but my husband loved the stuff...
I think that she gave some Natto to you.