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, or until it's fully cooked. Make sure the fish is 6-8 inches from the upper heating element. Keep an eye on it the last 5-10 minutes to ensure it doesn't get too charred. Turn the temperature down if the fish is caramelizing too quickly.
Notes
Marinate your fish as long as you can. Overnight is minimum but the longer the better.
A toaster oven works well with this dish. The compact size does a stellar job broiling the fish and developing a lovely caramelization.
Keep an eye on your fish for the last 5-10 minutes. The sugar in the marinade burns quickly. However, allow it to char slightly. Those caramelized bits are very tasty.
Add ginger to the marinade if you're using salmon. I typically don't use ginger with other fish but with salmon, it's a must!