In a large soup pot, heat the oil over medium-low. Saute the onions through the pepper flakes, stirring occasionally, until the onions become translucent; about 7-10 minutes. Then, add the garlic and saute another few minutes.
Add the broth and cauliflower and bring to a boil over high heat. Reduce to a simmer and allow to cook for about 15 minutes, until the cauliflower is tender. Remove from heat and transfer carefully to a blender (or use hand blender in pot). Blend on high (allowing steam to vent) for a few minutes, until silky and smooth.
Transfer back to the soup pot and stir in the coconut milk and vinegar. Bring back to heat over low, ensuring it doesn't boil.
Remove from heat and serve hot and topped with fresh dill and ground black pepper, if you like.
