Place the tiger prawns in a bowl. Using a spice grinder or small food processor, blend the chopped green Thai chilli, garlic, coriander stalks (snipped up with scissors), and caster sugar until smooth. Stir in half of the lime juice and the fish sauce, then pour this mixture over the prawns.
Heat 1 tablespoon of groundnut oil in a wok over high heat. Add the ginger and spring onions, frying for about 1 minute. Next, throw in the thinly sliced red pepper and cook for another minute until it starts to soften.
Add the water chestnuts and beansprouts, tossing everything just until the sprouts begin to wilt. Stir in the soy sauce and a good grind of black pepper, then transfer the veggies to a serving dish.
In the same wok, heat the remaining oil. Add the marinated prawns (lifting them out of their juices) and stir-fry for 1–2 minutes until they turn pink.
Return the remaining marinade to the wok, give everything a quick toss, then spoon the prawns over the veggies.
Sprinkle over the coriander leaves and squeeze in the rest of the lime juice. Serve your stir-fry over egg or rice noodles, with extra lime wedges on the side for an extra burst of flavour.