Use kitchen paper to dry the fish completely. Coat the fish with a thin layer of corn starch (see note 2).
Cut the fish at 45 degree angle several times on each side.
Add oil to a hot wok (or a frying pan). Place fish in the wok, then turn the heat down to medium. Flip it over when the first side is golden and firm. It takes roughly 2 mins for each side (see note 3).
While waiting for the fish to be fried, mix all the ingredients for the sauce.
When the second side is done, push the fish over to one side of the wok. Put scallions, ginger and garlic into the oil. Stir fry for half a minute or so.
Pour the sauce into the wok. Bring it to a boil then cover with a lid. Turn the heat to low. Leave to simmer for about 5 minutes each side. The sauce should be in a thick consistency at the end.
Scatter coriander shortly before dishing the fish out. Serve warm with plain rice.