Discard the leaves of the cauliflower. Cut the broccoli and cauliflower florets off their stem, them chop the stems finely.
Finely chop the onion and garlic.
Heat the oil over medium heat. Add the onions and cook for about 5 minutes until softened. Add the garlic and coriander stems, stir for a minute, then add the turmeric and curry powder and mix until the onion is well coated.
Pour in the stock and the coconut milk. Add the broccoli and cauliflower (including the stems). Bring to a boil, then reduce heat to low. Cover with a lid, and cook 15 to 20 minutes, until the vegetables are tender.
Using a stick blender or liquidiser, process the soup until roughly textured. Add the fresh herbs and season with salt and pepper to your liking. Blitz again until smooth and velvety.
Serve warm (reheat if necessary).