In a small bowl, mix together miso, mirin, sugar, soy sauce, toasted sesame oil, garlic and ginger. Place the fish on a rimmed baking sheet. Use a sheet that can fit in your fridge, because after you spread the sauce on the fish, you'll need to refrigerate it over night. Is this step absolutely necessary? Technically, no. But it does allow the flavors to really infuse into the fish. If you don't have the time to let it rest overnight, I highly recommend you let it sit, refrigerated, for at least a couple of hours.