Peel and de-vein the prawns, then halve lengthways.
Thinly slice white ends of spring onions into rounds and reserve. Chop green ends of onions and combine with oil in a small saucepan. Cook over low heat for 10 minutes to infuse. Cool to room temperature, then strain.
Dry-roast Sichuan peppercorns in a small frypan for 1-2 minutes, until fragrant. Finely grind using a mortar and pestle.
Finely chop 2 chillies. Using a mortar and pestle, pound chopped chillies with the Sichuan pepper, ginger, ¼ cup of coriander leaf, coriander stem, sugar and salt to a fine paste. Add vinegar and green onion oil, mix to combine and set aside.
Thinly slice remaining chilli. Combine in a bowl with prawns and sliced spring onions and remaining coriander. Drizzle with dressing, toss well to coat, arrange on a serving plate and serve immediately.