Step 1

Place the stock and water into a large pan over a high heat and bring to the boil.

Step 2

Add the lemongrass, coriander roots, galangal, tomatoes, lime leaves, lime juice, tamarind water and red chillies. Return to the boil, then reduce the heat and simmer for 15-20 minutes.

Step 3

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.

Step 4

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.

Step 5

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.