Bring the sake, mirin, and sugar to a boil in a small saucepan over high heat. Turn down the heat to a simmer, and simmer for 10 minutes to evaporate the alcohol. Keep the heat at low, and add the miso paste, whisking to combine. When the miso has dissolved completely, remove from heat. And cool to room temperature.

Dry the black cod fillets thoroughly with a paper towel. Slather the fish with the miso marinade and place in a non-reactive dish. Cover tightly with plastic wrap, and let marinate in the refrigerator for as little as 2 hours, and up to 3 days.

Preheat oven to 400° F. Heat an oven-proof skillet over high heat on the stove top. Add enough oil to film the bottom of the pan. Place the fish skin-side-up on the pan and cook until the bottom of the fish browns and blackens in spots, about 2 minutes. Flip fish and continue cooking until the other side is browned, roughly 2 minutes. Transfer to the oven and bake for 5 to 10 minutes, until fish is opaque and easily flakes with a fork.