Chilli and ginger dressing
1.Place green onions in a small bowl filled with iced water. Leave at room temperature until they curl.
2.Meanwhile, make chilli and ginger dressing: Dry-fry chilli in a small frying pan until blackened and puffed; cool. Using a mortar and pestle, pound chilli to a fine powder. Combine ground chilli with vinegar, sugar, salt and half the ginger in a small saucepan; stir over low heat, without boiling, until sugar is dissolved. Bring to the boil. Reduce heat; simmer, uncovered, without stirring, 5 minutes. Cool.
3.Place oysters on a serving platter. Drizzle with dressing; top with remaining ginger and the onion. Or, serve chilli and ginger dressing separately as a dipping sauce.