Make sure the fish is scaled and gutted ready for cooking. Lay the fish on a cutting board with the dark side up and make a couple of diagonal slashes or a cross with a sharp knife, cutting through the skin to the central bone. This makes the fish look attractive when served and helps it to absorb the simmering liquid evenly.
Cook the spinach in rapidly boiling salted water for 30 seconds to 1 minute, until it is wilted and bright green, Then plunge it into cold water to keep the color. Arrange the leaves neatly on a bamboo rolling mat and roll tightly to remove excess water. Set aside for a few minutes, then gently unroll from the mat and cut into 1 inch lengths.
Use a lidded saucepan large enough to lay the fish side by side, without overlapping. First put in the sake and mirin. Bring them nearly to the boil, then remove from the heat and ignite with a long handled match to burn off the alcohol. Shake the pan gently until the flame dies down. Add the dashi, soy sauces, and sugar and bring this stock back to the boil. (If you are worried about igniting the alcohol, simply mix all the stock ingredients together and bring to a simmer.)
Lay the fish, dark side uppermost, in a single layer in the pan. Cover with a drop lid (an ordinary lid will do), bring back to the boil, reduce the heat, and simmer over medium heat for 2 minutes. Baste the fish (do not turn it over) and simmer for another 2 to 3 minutes, basting frequently, until it flakes easily when tested with a fork.
Carefully lift the fish out of the simmering stock and arrange on shallow individual dishes. Pour over a little of the stock and garnish with a slice of rolled spinach. Serve hot or at room temperature