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Pat the fish dry with kitchen paper then cut into very big pieces. Put in a heatproof dish and evenly scatter with the ginger. Wrap tightly in cling film. Can be prepared to here up to 2-3 hours ahead, chill. Put the plate of fish into the microwave and cook for 3-5 minutes (depending on its thickness and how you like your fish cooked) on full power.
Remove the plate of cooked fish. Wait one minute and pour off any excess liquid that may have accumulated on the plate, then scatter the spring onions on the fish, together with the light and dark soy sauces.
Heat the two oils together in a small saucepan until very hot, then pour over the top of the fish. Scatter with a handful of coriander sprigs and serve at once.