Make a cross cut on the skin of the mackerel, sprinkle with sake, then leave it for about 5 minutes.
In a pot or a frying pan large enough for the mackerel to fit, put all the ingredients except for the mackerel, then turn on the heat.
Put in the mackerel once the sauce comes to a boil. Pour the sauce from time to time, and cook over a medium heat while removing the scum for about 8 minutes.
After turning the heat off, leave it for about 3 minutes.
Transfer the fish without breaking the filet on a plate, garnish with shredded shiso leaves or sesame seeds, then it's done.
Story Behind this Recipe
I used to cook this at my old workplace.
There's no need to add sugar as the pickled ginger already has sugar. You can replace the Tamari for normal soy however, Tamari gives a more mild flavor and brings out the umami (it'll take on a glossier sheen).