The origin given for it is Ishikari, on the Sea of Japan coast of Hokkaido. In the early Showa years, fishermen on the boats cooked the salmon they had caught on a sheet of iron cut from a drum, out at sea, and that is where this begins.

Salmon and whatever vegetables the season happens to provide go onto a hot plate with butter on top of them, and the whole thing is cooked under a lid, steaming at least as much as grilling, seasoned with miso, sugar, mirin, and sake.

Nobody knows where the name came from. The candidates are that it can be made chatcha to, quickly. That the father of the house, otochan, makes it. That the metal spatula against the iron plate makes a chan chan sound while you cook. All three explanations circulate locally and not one of them has ever won.

It was autumn and winter food, made when the salmon were running up the rivers, and it is now eaten right through the year. At the end, once everything on the plate has gone soft, you take the spatula and break the fish apart into the vegetables and mix the whole thing together. Something improvised on a deck with a piece of scrap metal is now a thing families do at a table.