Melt the butter over low heat and saute the onion and celery in it for a few minutes. Combine this with the stock and cauliflower in a large saucepan and simmer until the cauliflower is tender.
Use a slotted spoon to transfer the vegetables into a blender and then pour in as much of the broth as will fit. Puree the ingredients.
Pour the mixture back into the saucepan. Stir in the cream and add salt and pepper to taste.