Cut Saba into 8 pieces. Slice ginger thinly.

In a pot, heat water, sake, mirin, sugar, soy sauce, and ginger at medium heat. Add fish, lower to medium low heat, cover, and cook for 7-8 miunutes, turning fish once. Remove some of the sauce from pot and thin miso paste with it. Put the miso mixture back into the sauce in the pot. Cook uncovered for another 7-8 minutes or until the sauce has thickened a little.