1. Melt the butter or margarine in a large saucepan; add the onion and sauté gently for 5 minutes until soft. Add the cauliflower flowerets and potato slices. Cover and cook for 10 minutes.
2. Stir in the broth, bring to a boil, then cover and simmer for about 25 to 30 minutes or until all the vegetables are very soft.
3. Transfer to a blender and work until smooth. Return the purée to the rinsed-out saucepan and stir in the milk.
4. Heat the purée gently until simmering, then remove from heat and stir in the grated cheese. Season to taste with salt and pepper.
5. Pour the soup into individual soup bowls and swirl 1 tablespoon cream into each in a light feathery pattern over the entire surface of the soup. Place a quarter of the remaining cheese on each and sprinkle a little chopped parsley over the top.