Heat a knob of butter in a large saucepan and add 1 chopped onion, 1 sliced leek and 3 sliced celery sticks. Cook for 10 minutes until soft but not coloured.
Add the florets from 2 cauliflower heads, 500ml milk and 1 litre vegetable stock. Bring to a simmer and cook until tender. Add 150ml double cream and bring to the boil. Season with salt and grated nutmeg then blend until smooth.