- Put stock, 350ml water, coriander stalks, ginger, 3 lime leaves (reserve others for garnishing soup), chopped red chilli, lemongrass, and shallots in a saucepan. Bring to a gentle boil. Simmer for 10 minutes. Turn off heat and infuse for 10 minutes.
- Mix cashew butter, coconut cream and ¼ cup water together in a small bowl. Rinse fish, checking for scales. Cut remaining lime leaves into slivers, discarding centre rib of leaves.
- Scoop out and discard lemongrass, ginger and other flavourings. Get the broth very hot. Add fish sauce, lime juice, salt and mushrooms. Swirl in cashew butter and coconut cream. Add fish and cook until fish just turns white – about 3 minutes; the soup must not boil. Stir in sprouts and most of the coriander and sliced red chilli, and slivered lime leaves. Ladle into bowls. Garnish with coriander leaves and chilli and serve. To make it more substantial add a scoop of cooked rice to each bowl before topping up with hot soup.
Once the stock has come to the boil, lower the heat so the stock barely bubbles. Some brands of coconut cream tend to separate when heated with lime juice but this won’t affect the taste of the soup; shake the can well before use. Refrigerate any remaining coconut cream and use in a curry or rice dish.