Simmering liquid: I combined dashi (200ml), mirin (4 tbs), sake (4 tbs), red miso (3 tbs) and sugar (2 tbs) in a frying pan. I also added ginger root (4 thin slices). After coming to the boil, I turned the heat down tasted it. I thought it was too strong and added more water and sake but, in retrospect, I should have left it alone.
I placed the prepared mackerel in the pan skin side up and also added segments of scallion all around. After coming back to boil, I turned the heat down to simmer, I put an otoshi-buta and a regular lid (askew) and simmered it for 15-20 minutes (see image below).
To serve, I garnished it with thin threads of ginger ("hari-shouga" 針ショウガ) and scallion cut thinly along the long axis called "shiraga-negi" しらが葱 or white hair scallion. Since I do not have naganegi 長ネギ or Tokyo scallion, this was the best I could manage.
This is a classic and very comforting dish--perfect as an accompaniment for sake. Miso and mackerel are indeed a winning combination.