Place prawns, chilli, garlic, and lemongrass in a bowl. Add 1 tablespoon of the oil and toss well to combine. Marinate for 15 minutes.
Heat a large wok/pan over moderate heat. Add remaining oil and stir-fry prawns for 3-4 minutes until cooked, curled and pink. Transfer to a serving plate.
Add the lemon juice and wine to the pan and simmer until reduced by half.
Stir in the onion and herbs; season with salt and black pepper. Pour mixture over prawns.