Cut each fillet in half lengthwise, then cut widthwise in 3, making 12 pieces for the whole fish.
Combine [A] in a small bowl, put aside.
Heat sesame oil in the frying pan over medium heat until they become hot. Place the mackerel fillets skin side down, cook for 5 minutes. Skin should be slightly brown but not burnt. Reduce the heat if necessary.
Pour sake over the fish and bring to a quick boil. This will remove the fishiness of the mackerel. Then pour [A] over the fish. Add ginger and spring onion and simmer for 5 minutes over medium heat. Spoon the sauce over the fish several times. Sauce should be reduced to half the volume and it should be mildly thick.
To serve, place the fish in a bowl, spoon the miso sauce over, scatter the sesame seeds.