Put the olive oil, onion, and cauliflower in a soup pot over medium heat and cook until onion softens, then pour in the broth, bring to a boil, reduce heat to low, and simmer for 20 minutes or until cauliflower is very tender.
Remove pot from heat and let cool slightly.
When cooled, put mixture in a blender and puree until smooth, returning to a soup pot as you go.
When all the cauliflower is blended and in the pot, stir in the coconut milk, coriander, and salt, stirring to combine.
Turn heat to low under the pot and gently reheat, then taste and add more seasonings as needed.
Will serve 2 to 4.