In a medium saucepan, combine mirin, sake, and sugar. Stir over medium heat until sugar dissolves. Bring to a gentle boil and simmer for about 5 minutes until thickened.

Whisk in white miso paste until fully incorporated. Continue to simmer for 5-10 minutes until smooth and caramel in color. Allow to cool to room temperature.

Rinse black cod fillets under cold water and pat dry with paper towels. Place in a resealable bag in a single layer.

Pour cooled miso marinade over the fish in the bag, ensuring all surfaces are coated. Seal and refrigerate for 1 to 3 days.

Preheat oven to 450°F (232°C). Remove cod from the marinade and wipe off excess. Arrange on a lined baking sheet and bake for 5-8 minutes.

Switch oven to broil and broil for an additional 1-2 minutes watching closely to prevent burning. The top should be caramelized.

Transfer baked fish to a serving platter and serve hot over steamed rice, garnished with green onions or sesame seeds if desired.