Make cross cuts on the skin side of the Spanish mackerel fillets and pat dry with paper towels.

Rinse the komatsuna and place it on a microwave-safe plate. Cover loosely with plastic wrap and microwave at 600W for 2 minutes until softened. Rinse under cold water, squeeze out excess moisture, trim the roots slightly, and cut into 3cm pieces.

In a frying pan, combine sake, mirin, sugar, soy sauce, and water. Bring to a boil, then add the Spanish mackerel fillets skin-side up. Spoon the simmering sauce over the fish and cook over medium heat until the color changes. Cover with a drop lid and simmer on low heat for 5 minutes.

Dissolve the miso into the simmering sauce, cover with the drop lid again, and simmer for 10 minutes.