Season the fish with some salt and wrap it in a cheesecloth or paper towels.
In a bowl, combine miso, mirin, and sake to make miso marinade — miso doko or saikyozuke (西京漬け).
Spread some of the marinade on the bottom of nonreactive container. Place a layer of fish and cover with more miso doko.
Cover the container with a lid and keep in a refrigerator for 3-5 days.
Preheat the grill or oven to 200C/400F. Prepare some mirin in a bowl and a brush for glazing. Unwrap the paper towels with marinade and discard them.
Place the fish filletes on a roasting rack. Brush them with mirin and bake until cooked and the edges caramelized.
Serve with rice or soba.