Peel and finely chop the onion. Trim and finely chop the chili (removing the seeds if you prefer less heat). Peel and grate the ginger. Peel and finely chop the garlic cloves. Cut the cauliflower into bitesized chunks.
Heat 1 tbsp sunflower oil in a large saucepan and fry the onion for 5 minutes until soft.
Add the chili, ginger, garlic and fry for another minute.
Add the turmeric powder give it a stir, add the cauliflower and stir again. Add the coconut milk, palm sugar, salt and chicken stock.
Bring to the boil, cover and reduce the heat and cook for 25-30 minutes until the cauliflower is soft.
Use a stick blender to purée the soup. Season with the Thai fish sauce and the juice of one lime.
Serve the soup with a few fresh coriander leaves and a lime wedge.