Mix the prawns with 1 tablespoon of the olive oil a small bowl and set aside.
Soak the rice noodles in hot water to cover and leave for about 5–10 minutes until soft. Drain well, transfer to a serving plate and keep warm.
Make the lemon and fish sauce by mixing all the sauce ingredients in a bowl. (Any sauce not used in the recipe can be kept in an airtight jar in the refrigerator for a week.)
Heat a wok until very hot, add the oil-coated prawns and sear on each side for about 30 seconds until golden brown. Remove the prawns using a slotted spoon and set aside.
Heat the remaining oil, swirling it around to coat the wok. Add the garlic, chillies and lemon grass and stir-fry for about 30 seconds until the garlic is lightly browned. Toss in the onions and celery and stir-fry for a couple of minutes, until they have softened a little.
Return the prawns to the wok, add the shoyu sauce and spring onions and season with pepper. Arrange on top of the noodles and drizzle with 3 tablespoons of the lemon and fish sauce. Garnish with the crushed peanuts and the red chillies and serve with the remaining sauce in a jug.