Place a saucepan on medium heat and add the sake and mirin. Stir for 2-3 minutes and allow the mixture to come to a boil.

Add in the miso paste and stir continuously for 2-3 minutes until it has dissolved.

Add in half of the sugar or sweetener and stir. Add in the remaining sweetener and stir until it has dissolved. Taste the marinade repeatedly. Add additional sweetener to suit your taste if needed.

Place the fish in a shallow bowl or a zip log bag. Pat the fish dry and add the marinade to the fish. I like to use half of the sauce here for the marinade and the remaining to drizzle over the fish once it has cooked (for ultimate flavor).

Cover the bowl with wrap or foil or seal the zip lock and place the fish in the fridge. Marinate for at least 1 day, up to to 3 days.

Preheat oven to 400 degrees.

Place a skillet or grill pan (I use cast iron) on medium-high heat on the stove along with the butter.

Wipe away any excess marinade from the fish, but leave most of it intact. This is important because excess marinade will char, wiping the excess helps prevent that.

When the butter has melted, add the marinated fish to the pan with the fish skin side up.

Sear for 2-3 minutes until the fish turns brown. Flip the fish.

Place the fish in the oven and bake for 6-9 minutes.

Insert a fork into the thickest part of the fish. If the fish is cooked through, the flesh will separate easily and be opaque in color. You should also use a meat thermometer and test the internal temperature. The fish should reach an internal temperature of 135-145 degrees.

Cool before serving.