If desired, cut the fillets into 1 to 2-inch strips, about 2 to 3 oz. each.

Sprinkle salt all over the fillets and allow them to rest in the refrigerator for 30 minutes to 1 hour.

Remove the fillets and remove the salt by adding about 2 tbsp of sake all over them, using your hands to help get the salt off.

Meanwhile, place the mirin and sake (¼ cup) in a small saucepan over high heat and bring to a boil. Lower the heat and then whisk in the white miso paste. Bring to a boil again, and then turn off the heat. Stir in the sugar until completely dissolved. Transfer to a heatproof bowl and let sit until it cools completely to room temperature.

Place the fillets in a dish and pour the cooled marinade over them. Turn the fillets over a couple of times and use a spoon to help fully coat them. Turn them skin side up in the marinade and cover tightly with plastic wrap. Place in the refrigerator for 2 to 3 days.

Preheat your oven to 400°F.

Use a paper towel to rub off most of the marinade, however, do not rinse them.

Heat a skillet (preferably non-stick) over medium-high heat. Add a small amount of olive oil to the skillet.

Place the fillets, skin-side up, in the hot skillet. Sear them for 3 minutes until browned on the bottom, and then flip them and sear them for another 2 to 3 minutes.

Use a spatula to carefully transfer the fillets to a baking dish, preferably lined with a silicone matt. Place them in the oven until nicely browned, about 8 to 10 minutes. Remove from the oven and serve at once.