If using, finely chop the onion/leek and garlic.
Heat the olive oil or butter in a small saucepan over medium heat.
If using, add the chopped onion/leek and garlic and cook until softened, about 3-5 minutes.
Add the cauliflower florets and unsalted broth to the saucepan.
Bring to a boil, then reduce heat and simmer, covered, for 15-20 minutes, or until the cauliflower is very tender.
Carefully transfer the soup to a blender or use an immersion blender to blend until smooth and creamy.
Add a little more broth or cooled boiled water if needed to reach desired consistency.
Allow the soup to cool completely before serving.
