How to Make It
In a small saucepan, combine the water with the sugar and sake and cook over moderately low heat until the sugar dissolves. Whisk the miso paste into the marinade until smooth. Let the marinade cool completely.
Put the cod fillets in a baking dish large enough to hold them in a single layer and pour the marinade over them. Cover with plastic wrap and refrigerate for at least 1 hour or overnight.
Meanwhile, in a small saucepan, combine the mirin with the sake and 1 tablespoon of the ginger and bring to a simmer over moderately low heat. Cook for 5 minutes. In a small bowl, combine the water, miso and sugar and add to the sauce. Cook over low heat until the sauce is reduced to 2/3 cup, about 25 minutes; keep warm.
In another small saucepan, heat 1/2 inch of vegetable oil over moderate heat until shimmering. Add the remaining 1/4 cup of slivered ginger and fry until golden, about 30 seconds. Using a slotted spoon, quickly transfer the fried ginger to paper towels to drain; reserve the ginger cooking oil.
Preheat the broiler and arrange a rack 4 inches from the heat source. In a large skillet, heat 2 tablespoons of the ginger oil. Add the napa cabbage and stir-fry over moderately high heat until softened slightly and browned on the edges, about 5 minutes; season with salt.
Remove the cod from the marinade and pat dry. Set the fillets on a rack set over a baking sheet and broil for about 7 minutes, or until just cooked through. Spoon the cabbage onto plates and set the cod on top. Spoon a few tablespoons of sauce over the fish, garnish with the fried ginger and serve.