Use ready-to-eat rice vermicelli noodles, if you wish, in which case add them straight to the soup without rehydrating them first.
Put the oil in a large saucepan on a medium heat, add the curry leaves and let them sizzle for 30 seconds. Add the garlic, ginger and onion, then stir-fry for five to six minutes, until just softened.
Meanwhile, coarsely grind the coriander seeds and black peppercorns in a mortar, being careful not to overgrind them.
Add the spices to the pan, add a litre of boiling water, the tamarind paste and the sea salt, then bring to a boil. Lower the heat and leave to simmer for 10 minutes.
A couple of minutes before the soup is done, rehydrate the noodles in a bowl of boiling water for two minutes, then drain well and rinse under cold water.
Add the softened noodles to the soup, stir in the pak choi and prawns, and simmer for three minutes more, until the prawns are pink and cooked through.
Taste the broth, and add more salt as needed; the tamarind paste should have done the job on the sourness front, but brands vary, so stir in the juice of half a lime to add acidity, if need be.
Divide the soup and noodles between two bowls, top with the chilli, peanuts and coriander, and serve immediately with lime wedges alongside.
