Combine all the MARINADE ingredients in a bowl and whisk together until well incorporated. Be sure to work out any lumps of miso.
Put the fish and the marinade in a food storage bag and massage the marinade over all sides of the fish. Allow the fish to marinate at least overnight but 2+ days are ideal.
When you're ready to cook, take the fish out of the marinade and place it on a baking rack with a foil-lined baking sheet underneath to catch drippings.
Broil the fish in the oven or toaster oven at 400°F for 15-20 minutes. If your fish is extra thick, broil it for an additional 3-5 minutes. Keep an eye on it the last 5-10 minutes to ensure it doesn't get too charred. The sugar in the marinade burns fast. Turn the temperature down if the fish is caramelizing too quickly.