1. Gently pat the fillets dry. Don't wash the fishes.
2. Bring the sake, the mirin to a boil in a medium saucepan to evaporate the alcohol. Add miso paste and sugar and mix them with a wooden spoon. ( at medium heat ) Stirring constantly till mix well. Remove from the heat once the sugar is completely dissolved. Cool down to the room temperature.
3. Place some of the miso marinade on the bottom of a glass dish and lay the fillets on top of it.
Slather miso to cover on it. Let the fish marinade for 2 to 3 days in the fridge.
4. Preheat oven at 200C degree. Wipe excess miso marinade off of the fillets (don't rinse it off) and place the fish on the grill. Grill them until the surface of the fish turns brown. Then bake about 10 to 15 minutes.