Peel and de-vein the prawns, reserving the heads and shells. Peel and roughly chop the shallot, garlic and half of the galangal. Tear the lime leaves into small pieces. Bruise the lemongrass with the back of a knife.
Place a saucepan over a medium heat and add the sunflower oil, garlic, shallot, tomato purée, tamarind, galangal, lemongrass and lime leaves. Add the prawn heads and shells to the pan and fry until they turn red. Add the water to the pan, bring to the boil and simmer for no more than 15 minutes. Pass through a sieve to obtain the stock.
Finely chop the chilli. Pick the coriander leaves from the stalks - reserve the leaves for later and finely chop the stalks. Peel and finely chop the remaining galangal. Separate the leaves of the bok choi and rinse under cold water. Slice the spring onions thinly on the angle.
Place a wok over a high heat and add the peanut oil. Once the oil is hot, add the prawns, chilli and the galangal. Fry for 2 minutes and then add the coriander stalk, lime juice, bok choi, shrimp paste and fish sauce. Cook for a further minute and then add the stock to the wok. Bring to the boil, check the prawns are cooked and then remove from the heat.
Place a couple of bok choi leaves in the bottom of each bowl and add a small handful of fresh coriander leaves. Top with the prawns and then cover with the soup. Finish with the remaining coriander and spring onions.
«Add deep fried crispy shallots or chopped peanuts to this soup for extra texture and flavour.»