In a small bowl, combine miso, mirin, garlic and ginger.
Pour 1/4 of the marinade in a shallow dish or glass pie plate, place fish flesh-side down, and top with remaining marinade. Place in refrigerator and allow to marinade at least 6 hours or up to overnight.
Remove from refrigerator 30 minutes before cooking. Preheat oven to broil high.
Transfer fish to a sheet pan lined with foil. Discard excess marinade.
Place fish on the second rack from the top and broil 6-7 minutes, or until fish flakes easily with a fork.
If you wish to serve the fish without the skin, carefully slide a fish spatula between the skin and flesh.