Peel the prawns and reserve the heads and shells. Remove the back vein if necessary.
In a large saucepan, heat the oil and add the heads and shells. Cook over medium heat 2 minutes, then add the onion, shallots and carrot. Saute 2 more minutes.
Add the water, garlic, parsley, thyme and bay leaf. Bring to a boil then simmer 20-25 minutes, skimming any foam. Strain and discard the solids.
Bring the stock back to the boil and add lemongrass, ginger, chilli and coriander stems. Simmer ten minutes then strain again, discarding solids.
Bring the stock back to the boil. Add the spring onions and egg noodles. Simmer 4 minutes then add prawns, soy sauce, coriander leaves and lime juice. Simmer 3 minutes then serve in large bowls.