Add water and sake to a wide frying pan or shallow pot. Bring to a boil over medium-high heat. Let it boil for 30 seconds to cook off the alcohol.
Lower heat to medium. Place the fish in the pan, skin side up. The liquid should come halfway up the fish. Add the mirin, sugar, and ginger. Don't add the soy sauce yet.
Place a piece of aluminum foil directly on top of the fish as a drop lid (otoshibuta). Simmer on medium-low for 8–10 minutes.
Add 2 tbsp soy sauce, taste, and add more if needed. Continue simmering uncovered for another 5 minutes, spooning the liquid over the fish, until the sauce reduces into a glaze.
Carefully transfer the fish to a plate. Spoon the remaining glossy sauce over the top. Serve immediately with rice and miso soup.
