Combine mirin and sake in a saucepan on high heat. Boil for ~20 seconds, stir in the miso and sugar. Whisk over medium heat without letting the mixture boil until the sugar is dissolved and miso is mixed thoroughly.

Remove from heat and whisk in sesame oil. Allow to cool.

Pat the fish dry and place the fish and the miso marinade in a baking dish. Marinate for 2-3 hours or overnight if possible.

Preheat oven to 200°C. Cook fish brown on each side for 2 mins in a pan. Then bake for 10 mins.