Prepare the marinade (2-3 days in advance of serving):

Combine mirin and sake in small saucepan. Bring to a boil for approximately 20 seconds (allows the alcohol to burn off). Turn down the heat to low; add sugar, miso, and sesame oil and whisk until the sugar and miso dissolve. Remove from heat and let cool to room temperature.

Cut black cod filet into 4-5 pieces, slicing at a sharp angle. Place the fish in single layer in a baking dish, other container, or ziploc bag. When cool, add the marinade and turn to coat the fish. Cover/seal and refrigerate for 2 to 3 days.

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When ready to cook:

Remove the fish from the marinade and gently brush the marinde off with your hands, leaving a thin film. Do not rinse or wipe the marinade off. Place filets skin side down on a foil-lined baking sheet, and lightly season with kosher salt.

Arrange oven rack 6 to 8 inches from the top broiler. Set oven to broil (high). Broil fish, checking every few minutes and rotating pan as needed, until the fish starts to flake and the top is dark golden brown and beginning to char--approx. 8-12 minutes depending on the thickness of the fish.

Alternatively, you can sear the fish (skin side up) 2-3 mins in a med-hot skillet, carefully flip to skin-side-down for another 2 mins, then transfer to the oven to bake on middle rack at 400 degrees F for 5-10 mins. It's a bit trickier than broiling, but works just fine.

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Serve and enjoy. Delicious with Japanese rice and a stir-fried vegetables such as bok choy or sugar-snap or snow peas.

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