Step 1Whisk together the miso, water, beer, sugar, salt and pepper in a large nonreactive (glass or porcelain) bowl. Place the black cod in the miso mixture, making sure all sides of the fish are well covered with marinade. Cover and place in the refrigerator overnight or for at least 16 hours.
Step 2Pan roast the shallots in the olive oil over low heat until browned and caramelized, about 20 minutes. Set aside.
Step 3Remove the cod from the refrigerator. Heat the broiler on high. Line a saute pan with aluminum foil and generously brush the foil with butter. Place the cod on the foil, skin side down, making sure not to remove all the miso marinade. The cod pieces should have a light coating of marinade all around and not touch each other.
Step 4Place the pan directly under the broiler. There should be about 1 1/2 inches between the fish and the heating element. Broil the cod for 7 to 10 minutes or until browned and cooked (the fish will appear flaky and soft). If the fish browns too fast, either reduce the temperature or, even better, lower the pan so it's farther from the heating element.
Step 5Serve the cod immediately with the roasted shallots and frisee for garnish and texture. Sprinkle the finished dish with bonita flakes.