Preheat a broiler (or grill), setting the rack as close as possible to the heat source. Put the miso in a bowl, add the mirin, and whisk until smooth. Stir in the onion, a little bit of salt, and a pinch of cayenne. Add the scallops and marinate while the broiler or grill preheats, or refrigerate for up to a day.
Broil until lightly browned, without turning, for 2 to 3 minutes, or grill, turning once after a minute or 2. Sprinkle with the lime juice and serve.
You can also marinate sturdy white fillets this way—catfish, blackfish, monkfish, red snapper, and grouper are all good examples—or the old standby, salmon. Like the scallops, they should not be turned if you’re using the broiler. They should, however, be placed about 4 inches from the heat source so that the coating does not burn before the fish cooks through.
