Scrub the oysters clean, then divide between 2 heatproof plates; you will have to steam them in 2 batches. Set up a steamer, or put a rack in a lidded wok or deep pan and fill with 5cm water.
Bring to the boil over a high heat. Put 1 plate of oysters in the steamer or on the rack and cover with the lid. Turn the heat to low and gently steam the oysters for 5 minutes or until they begin to open.
Meanwhile, trim and finely shred the spring onions, peel and finely chop the ginger and garlic, and deseed and finely chop the chilli, then place in a heatproof bowl. Add the remaining sauce ingredients, except for the oil.
Heat a wok or large frying pan over a high heat until hot. Heat the oil until slightly smoking, then pour it over the sauce.
Remove the first batch of oysters from the steamer. Give the sauce several good stirs. Carefully remove the top shells of the oysters and drizzle a bit of sauce over each one.
Garnish with coriander leaves and serve at once. Meanwhile, repeat the cooking procedure with the second batch.