Make the marinade (2–3 days in advance): In a small saucepan over high heat, bring ¼ cup sake and ¼ cup mirin to a boil. Let boil for 20 seconds to burn off the alcohol.

Lower the heat and whisk in ¼ cup white miso paste and 3 tablespoon granulated sugar until smooth. Let cool to room temperature.

Marinate the fish: Pat the cod fillets dry with paper towels. Place in a zip-top bag or shallow container. Pour the cooled marinade over the fish and coat evenly. Cover and refrigerate for 2–3 days.

Broil the cod: When ready to cook, preheat the oven to broil with the rack 6–8 inches below the heating element.

Line a wire rack or baking tray with foil and lightly spray with cooking spray.

Remove the cod from the marinade, gently brushing off excess but leaving a thin layer on. Lightly season with kosher salt.

Place fish on the prepared tray, skin-side down (or smooth side down if skinless).

Broil for 8–12 minutes, rotating the tray halfway through, until the top is golden, bubbling, and just starting to char. The fish should flake easily and appear glossy.

Serve immediately, optionally garnished with scallions, sesame, or lemon.