One or two hours before serving, sprinkle the fish with salt and refrigerate.
Place the parsley, mushrooms and fish in a dish that will fit into a bamboo or other steamer. Sprinkle fish with sake and steam for 10 to 15 minutes.
To make Ponza Sauce, combine ingredients in a small bowl and mix thoroughly.
Serve cooked fish with Ponza Sauce on the side for dipping.
Serve with steamed broccoli and rice.