Miso tends to burn when broiling, so be very careful that the fish is not close to the heat source. Also do not forget to oil the grill with a mild oil such as rapeseed. The miso dries out on the skin, giving the fish a slight crust and a moist center.
Set the oven rack in the third slot from the top and preheat the broiler.
Measure the miso into a small bowl and whisk in the sake. Lay the cod filets on a cutting board or plate. Spread the miso mixture evenly over the surface of both sides of the fish.
Transfer the filets, skin side down, onto an oiled fish rack, and perch on a cookie sheet to catch any drips. Broil for 4 minutes, flip gingerly (the miso sticks), and broil the skin side for 3 minutes. Carefully remove the filets from the rack and serve immediately or at room temperature.
Variations: Salmon filets would also be a good choice. Barbecuing will result in a more flavorful fish but take care that the coals are at a very low ember.