First, prepare the stock. Remove the shells from the prawns and set the prawns aside. Place the shells in a small pan and cover with water. Add the lime leaves and peppercorns, then simmer over a gentle heat for five minutes.
For the spice paste, place the garlic, lemongrass, ginger and chilli into a small food processor and blend for a few seconds. Add the coriander and blend to a rough paste.
Strain the prawn stock through a sieve, discard the shells, then return the stock to the pan and keep warm.
For the prawns, pour the oil into a large, heavy-based saucepan over a medium heat, then add the spice paste. Fry gently for two minutes then add 300ml/½ pint of the stock.
Add the coconut milk, a generous pinch of salt, the mangetout and the prawns. Simmer for 3-4 minutes.
Divide the beansprouts between two large bowls, then spoon the prawn curry over the top, allowing the beansprouts to cook in the residual heat. Serve immediately.