For the dressing, mix the lime juice, brown sugar, fish sauce, and minced garlic in a small bowl; set it aside. In a large mixing bowl, pour boiling water over the rice noodles. Let them sit for about 2 minutes, or until they're just tender, then drain them well under cold water. Shake the sieve several times to get rid of any extra water, then return the noodles to the bowl.
Add the prawns, thinly sliced red onion, chopped red chilli, cucumber, and a generous handful of both coriander and mint to the noodles. Pour the dressing over everything and toss to combine. Finish by sprinkling the crispy shallots and roasted peanuts over the top.