First, make the nam jim dressing. Finely grate the lime zest into a food processor, and squeeze in the juice. Deseed the chilli, roughly chop the coriander and peel and roughly chop the ginger, then add to the processor.
Drizzle over the fish sauce, then add the palm sugar, and blitz until finely chopped. Season to taste and set aside.
Pour 3cm of sunflower oil into a wide, deep saucepan and place over a medium heat until the oil reaches 180ºC on a thermometer. You can also test that the oil is hot enough with a little bit of noodle – it should crisp up and turn lightly golden.
Meanwhile, break up the noodles into 5cm strips. Prepare a plate lined with kitchen paper and have a slotted spoon ready. Fry the noodles in the oil, in batches for 30 seconds at a time, turning halfway through, then transfer to your lined plate to drain.
Heat 1 tablespoon of groundnut oil in a large frying pan over a medium–high heat, add the prawns and fry for 4 minutes, turning halfway through, or until cooked through and lightly golden – you may need to work in batches.
Tip the prawns onto a serving dish, drizzle over the nam jim dressing and toss to combine. Trim and finely slice the spring onions.
To serve, sprinkle the crispy noodles and spring onions over the prawns, then bring to the table with lots of forks and let people dig in.