Heat oil in a large pot over medium heat until shimmering.
Cook the onions and 1/4 teaspoon salt until onions are soft and translucent, 8 to 9 minutes.
Reduce heat to low, add garlic, and cook for 2 additional minutes.
Add cauliflower, broth or water, coriander, turmeric, cumin, and remaining 1/4 teaspoon salt.
Bring to a boil over medium-high heat, then reduce the heat to low.
Simmer until cauliflower is fork-tender, about 15 minutes.
Working in batches, purée the soup in a blender until smooth and then return the soup to the soup pot. (Alternatively, use an immersion blender to purée the soup right in the pot.)
Stir in the coconut milk and warm the soup. Taste and add more salt, pepper, or spices if you’d like.