Melt the butter in a saucepan. Add the onion and sweat for 1-2 minutes until softened.
Add the cauliflower and enough water to just cover the ingredients. Stir to combine, bring to the boil and reduce to a simmer for 8-10 minutes or until tender.
Drain through a colander and place the cauliflower mixture in a food processor.
Add the cheese and cream and blend until you have a thick puree.
Season with salt, freshly ground white pepper and pinch of nutmeg.