Make 4 deep, diagonal slashes down to the bone on both sides of the fish. Insert the ginger into the slashes. Refrigerate the fish until ready to cook.
In a small saucepan, bring the sake, soy sauce, 1/4 cup lemon juice, and sugar to a boil. Remove the pan from the heat, and let it cool. Stir in the sliced scallions.
Fill a wok or deep-fryer large enough to hold the fish with peanut oil to cover the fish by 1 inch. Heat the oil until very hot, about 375 degrees F on a deep-frying thermometer.
Season the fish lightly all over with salt and pepper, then dust it lightly but evenly with cornstarch. Hold the fish by the tail, carefully slip it head first and away from you into the hot oil. Fry it until the skin is crispy and golden brown and the flesh is cooked through, flaky, and separates easily from the bone, about 10 minutes.
With long-handled metal tongs, remove the fish from the wok, letting excess oil drip back into the wok. Transfer the fish to a serving platter. Garnish the fish with scallion strips and cilantro and squeeze the lemon half over it. Serve immediately, passing the sauce on the side.