Rinse the black cod and pat it dry thoroughly with paper towels.
Mix the white miso and sake until smooth. Spread the mixture evenly over the surface of the fish. Cover with plastic wrap and refrigerate overnight.
Preheat the oven to 190°C (375°F) and position the rack in the middle of the oven.
Remove the fish from the refrigerator and wipe off all the miso from the surface using paper towels.
Heat a small amount of oil in an oven-safe skillet over medium heat.
When the oil begins to lightly smoke, place the fish skin-side up (flesh-side down) into the pan. Cook over medium-low heat for about 2–3 minutes, until the surface is nicely browned.
Flip the fish, then transfer the skillet directly into the oven. Bake for about 15–20 minutes, until the internal temperature reaches at least 63°C (145°F). Serve immediately.
White miso is recommended for this dish. Its mild, lightly sweet flavor is better suited for fish than red miso.
The black cod can be marinated in the refrigerator for 1–3 days. The longer it marinates, the saltier the flavor will become.
Baking time depends on the actual oven temperature, the thickness of the fish, and how long it has warmed up after being removed from the refrigerator. If you don’t have a thermometer and are unsure whether the fish is fully cooked, bake it a bit longer. Black cod is rich in fat and naturally tender, so slightly longer cooking will not dry it out.
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