Steps

1. Thinly slice 20-30g of ginger against the grain. This is key! Slicing against the grain helps release more ginger flavor.
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2. Make about 2 diagonal slits on the skin side of the mackerel fillets. This is key! It helps them cook evenly, prevents the skin from bursting, and allows the flavor to penetrate better.
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3. Place the mackerel fillets in a bowl and pour hot water over them. Stir with chopsticks to ensure the hot water reaches all parts.
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4. Discard the hot water and rinse with cold water. Once cool enough to touch, remove any surface impurities and blood clots from the underside.
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5. In a large pot or pan, combine 3.5 tbsp sugar, 100ml cooking sake (rice wine), 300ml water, and 20-30g thinly sliced ginger.
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6. Stir lightly to dissolve the sugar.
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7. Bring to a boil over medium heat.
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8. Measure out 5 tbsp miso into a small bowl.
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9. Once the liquid boils, carefully place the mackerel fillets into the pot, skin-side up.
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10. Skim off any scum that rises to the surface with a ladle.
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11. If there isn't much scum, turn off the heat temporarily.
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12. Take about one ladleful of the simmering liquid from the pot and add it to the bowl of miso. Mix the miso lightly to dissolve it.
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13. Pour the dissolved miso mixture back into the pot. Use the simmering liquid to fully incorporate the miso without lumps. This is key! It helps the miso dissolve easily and prevents the mackerel from breaking apart.
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14. Wait for the mixture to come to a boil again.
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15. Once boiling, cover with a drop lid (otoshifuta) and reduce the heat slightly. Simmer the mackerel fillets for 5-6 minutes.
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16. When the mackerel fillets are cooked through, turn off the heat.
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17. If you have time, carefully remove the mackerel fillets from the pot.
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18. Let them cool in the simmering liquid for about 1 hour. This is key! Braised dishes absorb flavor as they cool, so this step allows the miso flavor to deeply penetrate the mackerel.
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19. After cooling, return the pot to medium heat and reduce the simmering liquid.
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20. Once boiling, cover again with a drop lid, reduce the heat slightly, and simmer for about 10 minutes to further reduce the liquid.
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21. After 10 minutes, remove the drop lid.
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22. Continue simmering for about 2 to 4 minutes, monitoring the consistency and heat, until the sauce reaches your desired thickness and richness.
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23. The sauce should be reduced until it coats the bottom of the pot with a thick, syrupy consistency. Turn off the heat.
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24. Plate the mackerel, spoon the remaining sauce over it, and garnish with shredded leek, if desired.
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