Two to 3 days beforehand, make the miso marinade. Bring sake and mirin to a boil in a small saucepan, boil for 20 seconds to evaporate alcohol. Reduce heat to low, whisk in white miso paste and granulated sugar until dissolved. Remove from heat and cool to room temperature.

Pat the black cod fillets dry with paper towels. Place in a zip-top bag or container that fits the fish snugly. Add marinade and coat the fish. Seal and refrigerate for 2 to 3 days.

When ready to cook, remove fish from marinade, gently brush off excess marinade, and lightly season with kosher salt.

To broil: Arrange rack 6 to 8 inches from the broiler and heat oven. Cover wire rack with foil and spray. Place fish skin-side down and broil for 8 to 12 minutes until flaky and golden.

To stovetop sear and bake: Preheat oven to 400°F. Heat oil in oven-proof skillet over medium-high heat. Sear fish skin-side-up until browned, about 2 to 3 minutes per side. Transfer skillet to oven and bake for 5 to 10 minutes until opaque and flakes easily.

After cooking, transfer fish to plates with a thin metal spatula and check for pin bones before serving.