Bring water to a boil in a pan. Have ice water ready in a bowl. Blanch one fillet for 30 seconds and immediately plunge it into the ice water. Repeat with the other fillet. Pat the mackerel dry with paper towels.
Combine water, sake, sugar into a pan and bring to a boil, then put in ginger and the mackerels with the skin side up and cook over medium-high heat. Remove the scum.
Turn off the heat once. Dissolve miso in a ladleful of the step ②'s soup in the bowl. Put the miso back in to the pan, turn on the medium heat again, once boiled, after that lower the heat and simmer until the liquid is almost gone, about 10 mins.