Combine the mirin and sake in a small saucepan over high heat. Bring to a boil. Turn heat to low and whisk in the miso until dissolved. Turn the heat back up and add the sugar. Stir in until the sugar completely melts, whisking quickly to assure the sugar doesn’t burn at the bottom. Transfer to a glass dish. Let cool to room temperature, then add fish and turn to coat. Cover snugly with a lid or wrap and refrigerate at least overnight, but preferably three nights.
When ready to cook, preheat oven to 400°F/200°C. Prepare a heavy rimmed baking sheet by greasing it with a light coat of vegetable or avocado oil. Set aside.
Heat a tablespoon or so of oil in a large pan over medium-high heat. Wipe the marinade off the fish. If your fish has skin, put into the pan flesh side down. When the oil shimmers and starts to smoke lightly, add the fish. Give it a shake to assure it doesn’t stick. Cook until browned and a white line rises from the bottom, about 2 minutes. Flip the fish onto the prepared pan and put into the hot oven to roast for 8 minutes. It’s done when it just begins to flake and it’s cooked through to the center; an instant-read thermometer should read 140°F/60°C.