75g/3oz palm sugar (available from Asian supermarkets), use brown sugar if unavailable
12 raw tiger prawns, shelled, gutted and split in half
2 boneless skinless chicken breasts, cut into chunks
1 small bunch fresh coriander, leaves
1 small bunch fresh Thai basil, leaves (available from Asian supermarkets)
Place the stock and water into a large pan over a high heat and bring to the boil.
Add the lemongrass, coriander roots, galangal, tomatoes, lime leaves, lime juice and red chilies. Return to the boil, then reduce the heat and simmer for 15-20 minutes.
Taste the stock and add as much fish sauce and palm sugar as necessary to keep the salty-sour-hot-sweet taste, then remove from the heat.
Transfer 220ml/8fl oz of the stock into a separate pan over a high heat and bring to the boil. Add the chicken and cook for 1-2 minutes, then add the prawns and cook for a further 1-2 minutes, or until completely cooked through.
To serve, pour the soup into two bowls and top up with the hot stock. Garnish with Thai basil, coriander and a wedge of lime.