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1
Combine the white miso, sugar, and mirin in a bowl and mix to a uniform paste. The paste should be smooth, cohesive, and glossy — no dry pockets of miso remaining.
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2
Coat each fish fillet thoroughly on all sides with the miso paste. Place in a non-reactive container, cover, and refrigerate for a minimum of 24 hours — 48 to 72 hours is preferable. This is where the dish lives or dies: the miso enzymes require time to penetrate and tenderise the flesh; a short marination yields sweet fish, the full marination yields transformed fish.
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3
Remove the fillets from the refrigerator. Using a clean cloth or paper, wipe away the excess miso paste from the surface. The fish should be lightly coated — not bare, not thickly crusted. Residual paste left in excess will burn before the fish is cooked through.
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4
Position the oven rack close to the broiler element and heat the broiler to its highest setting. Lay the fillets on a lightly oiled rack or lined tray and broil until the surface caramelises to a deep golden-brown. The aroma will shift from fermented and savoury to sweet, umami-rich, and caramelised miso — this is the cue that the glaze is ready.
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5
Remove the fillets when the surface is uniformly deep golden-brown and the flesh yields to gentle pressure — silky and tender within, sticky-sweet and lacquered at the surface. Rest briefly before serving.