For the cauliflower puree, peel and finely chop the onion. Divide cauliflower into small florets and rinse. Melt butter in a saucepan and sauté onion until translucent.
Add the cauliflower and sauté for about 3 minutes until colorless. Season with salt and pepper. Pour in water and simmer for about 5 minutes (I usually leave it until the water is greatly reduced).
Pour off water (amount depending on how solid or liquid you would like the puree to be) and then puree the whole thing with a hand blender.
Stir in créme fraîche and butter, season with nutmeg and salt if needed. If necessary, puree the cauliflower puree again with a hand blender until creamy.