Pat the fish fillets dry with kitchen paper if needed, and season. Put on a heatproof plate and scatter over the shredded ginger.
Pour water into a wok or deep pan to a depth of 2.5-5cm (1-2in) and insert a steaming rack. If you don't have a steaming rack, use an upturned heatproof dish to make a trivet. Put the pan on a high heat and bring the water to the boil.
Put the plate of fish on the steaming rack or heatproof dish. Cover with a lid and gently steam for 10-12 minutes, until the fish is opaque and flaky.
Remove the fish from the wok and divide between 4 plates. Drizzle over the light and dark soy sauces and scatter over the spring onions.
Heat the oil in a small pan until smoking hot and carefully pour over each piece of fish. It will sizzle a little. Scatter over the coriander and serve immediately.