Combine the ginger, garlic and lemon zest in a small bowl. Lay the fish, skinned side up, on a work surface and sprinkle with salt and the ginger mixture. Fold the fillets in half widthwise. Place the folded fish on a heatproof plate or a shallow dish and drizzle the sesame oil over the top.
Sit a steamer over a wok filled with boiling water, ensuring that the steamer is large enough to fit the plate and that the water does not reach the steamer.
Put the plate with the fish in the steamer, cover tightly and steam for 10 minutes, or until the fish is just cooked. It should look opaque and flake lightly but still remain moist. Using a spatula, transfer the fish to a serving dish and cover loosely with foil to keep warm. Reserve any juices that are left on the plate.
Pour these juices into a small saucepan. Add the lemon juice, cornflour paste and 100 ml of water. Bring to the boil, stirring, and cook for 1 minute until the sauce is lightly thickened. Stir in the chopped coriander.
Drizzle the sauce over the fish and scatter with spring onions.