Place the butter, shallots and celery in a saucepan and fry gently for 5 minutes without browning. Add the cauliflower florets, mustard, bay leaves, thyme and stock. Bring to the boil and simmer for 20 minutes. Remove the bay leaves and thyme sprigs, and blend until smooth. Season with salt, pepper and nutmeg, and stir in the cream. Garnish with slices of Brie and chopped chives, and serve.