Melt fat in cast iron pot over medium heat. Add onions and salt. Sweat over low heat until they are soft and clear. Make sure not to brown the onions.
Add cauliflower, saute for 10 minutes, then add water and bring to a boil. Once boiling, lower to a simmer and let cook partially covered for 20 minutes.
Transfer to a blender and emulsify until smooth. While blender is on high, add butter to thicken the soup.
Transfer back to pot and season with fresh black pepper