Set up a steamer: Add 2 inches of water to a wok or large pot and bring to a rolling boil over high heat. Set a steaming rack inside, ensuring a heatproof plate fits on top without touching the water.
Prep the fish and aromatics: Pat the fish dry. Make 2–3 shallow diagonal slashes on each side to the bone. Julienne the ginger and scallions into thin matchsticks.
Season and plate: Sprinkle the fish evenly with the salt. Scatter half the ginger and about one-third of the scallions on the heatproof plate to form a bed. Lay the fish on top and tuck a little of the ginger into the cavity. Drizzle the Shaoxing wine over the fish.
Steam: Carefully set the plate on the rack, cover tightly, and steam over high heat for 8–10 minutes, until the thickest part of the flesh is opaque and flakes easily and the dorsal fin pulls out with gentle tugging. (Add 1–2 minutes if your fish is thicker.)
Make the sauce while the fish steams: In a small saucepan, combine the light soy sauce, sugar, and water. Heat over medium until hot and the sugar dissolves, 1–2 minutes. Keep hot off heat.
Heat the finishing oil: In a separate small saucepan, heat the peanut oil over medium-high until shimmering and just beginning to smoke, 1–2 minutes. Remove from heat.
Drain and top: When the fish is done, carefully remove the plate from the steamer. Tilt off and discard the watery juices from the plate and remove any spent ginger. Mound the remaining ginger and the rest of the scallions evenly over the fish.
Sizzle: Immediately pour the hot peanut oil over the ginger and scallions so they sizzle and release their aroma.
Sauce: Drizzle the hot soy mixture around the fish and a little over the top to season without washing off the aromatics.
Finish and serve: Scatter the cilantro leaves on top and serve right away.