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1
Combine the sake and mirin in a small saucepan over medium heat. Bring briefly to a simmer to evaporate the alcohol — the sharp raw spirit smell will soften to a rounder, sweeter vapour. Remove from the heat and leave to cool completely.
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2
Whisk the saikyo shiro miso into the cooled sake-mirin mixture until fully dissolved and uniform — the marinade should be a smooth, pale-gold paste of flowing consistency, neither grainy nor pooled with liquid.
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3
Coat the cod fillets thoroughly in the marinade, packing it against all surfaces. Seal in a non-reactive container and refrigerate for a minimum of 48 hours; 5–7 days produces the deepest penetration of the miso into the fat-rich flesh. The fish should be fully enveloped throughout.
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4
Remove the fillets from the marinade. Using paper towels, wipe the surface clean of excess miso — this is where the dish lives or dies: residual miso sugar will char to black carbon within seconds under heat, before the fish is cooked through. The surface should appear lightly filmed, not thickly coated.
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5
Position the fillets skin-side down on a lightly oiled rack beneath a salamander or overhead broiler set to 180–190°C. Grill until the flesh is cooked through to an internal temperature of 60–62°C — at this point the high fat content renders the interior buttery and yielding, not flaking dry. The surface will deepen steadily to amber.
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6
In the final 30 seconds, blast under maximum heat, watching constantly. The exterior should reach a burnished, lacquered mahogany — glossy and set, with a faint char at the thinnest edges. Remove the instant the surface is uniformly caramelised; the miso sugars pass from lacquered to carbonised in moments.
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7
Plate immediately with pickled ginger and a lemon wedge. If using the heated saikyo miso sauce, warm a small quantity of the reserved marinade gently and spoon alongside. The surface should hold its gloss to the table.