Peel the prawns, leaving the tails intact, and gently pull out the dark vein from each prawn back, starting at the head end. Keep the shells, and heads. Heat the oil in a large stockpot or wok and add the prawn heads and shells. Cook for 5 minutes, or until the prawn shells turn bright orange.
Add one lemongrass stem to the pot with the galangal and stock or water. Bring to the boil, then reduce the heat and simmer for 20 minutes. Strain the stock and return to the pot. Discard the shells and flavourings.
Finely slice the remaining lemongrass and add it to the liquid with the chillies, lime leaves, fish sauce, mushrooms and spring onions. Cook gently for 2 minutes.
Add the prawns and cook for 3 minutes, or until the prawns are firm and pink. Take off the heat and add the lime juice. Taste, then adjust the seasoning with extra lime juice or fish sauce if necessary. Garnish with coriander leaves.