Pat dry your fish and place it on a parchment paper-lined sheet pan.
In a saucepan, combine the mirin and sake. Bring the liquid to a boil and then lower to medium heat.
Add the inaka or red miso and combine thoroughly until the miso is dissolved.
After one minute, add sugar and continue to cook for about two to five minutes or until the mixture is the same consistency as honey. Stir throughout this thickening to make sure the sugar doesn’t crystalize on the sides or burn at the bottom of the pan. Remove the marinade from the pan and let sit to cool for about 10-15 minutes (it should also thicken after cooling).
In a glass container, place the filets and thoroughly coat with cooled marinade. Leave to marinate skin-side up for up to 30 minutes on the counter.
Preheat your oven to 400 °F on broil and place a rack in the second tier of your oven (this is about six inches away from the broiler in my oven).
After marinating, place the fish skin-side down on a parchment paper-lined sheet pan and put the pan in the oven.
Broil for about five minutes and then check the temperature or progress of your fish. Since my filets are about an inch thick, they measured to be about 110 °F at the five-minute point. To get a hotter broil over the glaze, move the rack and filets to the top rack of the oven and broil for an additional two to four minutes. Broiling your fish can take about 7-9 minutes, but use a thermometer to measure the doneness of your fish if you are worried because it will be based on the thickness of your filets and also your preference. I pull my fish around 125-130 °F since it will finish cooking outside of the oven. Fish is technically fully cooked at 145 °F.
Serve with a side of rice and bok choy.
Notes
Wild salmon will cook faster in the oven because it's usually leaner and thinner, adjust your broiling time based on this and the thickness of your filets.