Cut the mackerel and score the fish meat (on the skin side) so that it will not break apart.
Pour boiling water over the mackerel fillet until the flesh turns slightly white then move it to a bowl of ice water to prevent further cooking and to remove any blood clots.
Making Saba Misoni
Slice the ginger very thinly and cut the scallions. Set both aside.
Put all the sauce ingredients into a shallow sauce pan or a pot over medium heat and bring to boil.
Turn the heat down to low and add the mackerel gently into the pan.
Put a drop lid on and simmer for about 10 minutes to reduce the sauce but be careful not to burn it.
While simmering the fish, occasionally spoon the sauce over the fish.
Garnish with ginger and spring onion to serve.
Notes
Other fish options- sardines, salmon fillets, and horse mackerel are good too.