https://satwcomic.com/nordics-like-fish
“Surströmming” is "fermented" fish and, essentially, calling it "rotten" is somewhat accurate, it's sealed into airtight container so that only anaerobic bacteria can decompose certain parts of it, resulting in something that humans can still eat.
Some legends claim that it was invented when Swedish fishermen ran out of salt and sold their extra fish to Finns without it, sealing the containers to disguise the fact as long as possible. Somehow they made contact again and Finns asked to buy more.
("Finns" in this case could refer to Swedish settlers in Finnish coast, and idea of someone trying to cheap-skate on salt sounds somewhat plausible, I think "sell the crap to dumb Finns, they ask for more and crap turns out to be good" is likely an later invention to explain it.)