In a medium saucepan, combine the brown sugar, mirin and sake and bring to a simmer over moderate heat. Reduce heat to low, stir in miso to blend, and cook 8 minutes, stirring occasionally. Set aside to cool.
Spread a third of the miso marinade in a glass baking dish large enough to hold the fish in a single layer. Arrange the fish, skin-side down, and spread the remaining marinade on top to completely coat them. Cover with plastic wrap, and refrigerate for a minimum of 12 hours. You can keep refrigerated as long as 24 hours, but not longer.
Light a grill or preheat the broiler. Scrape the marinade off the fish, and pat the fish dry with paper towels. Throw away the marinade.
Brush fish with olive oil. Grill or broil the fish (8 inches from the heat source for a broiler; 6 inches from the heat for a grill) for about 6 to 8 minutes, turning once. The fish will caramelize as it cooks.