Remove scales, gills, and guts from the fish. Rinse with water and pat dry with paper towels. Score the skin with cross cuts and season with sake and salt.

Slice the ginger thinly. Cut the green onion lengthwise, remove the core, and julienne. Soak the julienned green onion in water and drain.

Place kombu, fish, ginger, green onion core, sake, and water in a pan. Heat until boiling, then cover and simmer on low heat for 15 minutes.