1.Descale the fish, if the skin has scales. Sprinkle salt over both sides of each fillet and set aside for 30 minutes.
2.Meanwhile, in a small bowl add miso, mirin, 3 tablespoons sake, and sugar. Whisk until fully combined. Pour the marinade into a flat bottom container that will hold all the fish.
3.Pour 2 tablespoons sake over the fish to use it to rinse off the salt. Gently pat dry with paper towel. Do not rinse fish under water. Place fish into container with marinade and turn fish around a few times to coat. Make sure all sides are coated with marinade. Cover with a lid and place in the fridge overnight.
4.Preheat oven to 425 degrees. Prepare a baking sheet lined with parchment paper.
5.Take the fish out of the fridge and with your fingers remove marinade completely. Place fish skin side up on prepared baking sheet. Bake on center rack until the surface is blistered and brown a bit, 15-20 minutes (no need to flip).
6.Remove the fish with a spatula and remove burnt miso around fish. Serve immediately.