In a medium pot or Dutch oven, combine the cauliflower, water and onion. Cook, covered, over medium heat until the cauliflower is tender, 20 to 30 minutes.
Let the soup cool slightly, then place it in a blender with the cheese and a half teaspoon of the salt. Purée until smooth and creamy. (Bring the speed up slowly for best results.)
Carefully remove lid and return the soup to the pot. Over low heat, warm through and add more salt and pepper to taste.
Garnish with a sprig of thyme or pan-roasted cauliflower florets.
