Pour the stock and water into a large saucepan and bring to the boil. Add the lemon grass, coriander root, galangal or ginger, tomatoes, lime leaves, the juice of one lime, and the chillies. Turn the heat down slightly and simmer gently for 30 minutes, until the liquid has reduced slightly and the flavours have infused.
Strain the liquid into another pan and add the chicken. Simmer for 5 minutes. In the last minute of cooking, add the prawns, mushrooms, fish sauce and palm sugar. Add more fish sauce – the soup should be a balance or sour, hot and sweet. Ensure chicken is cooked through with no pink showing.
Serve sprinkled with the coriander leaves and with a wedge of lime.
*Inspired by Brian T. featured in the Realfood Cookbook
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.