Wash, trim and cut cauliflower into florets. Melt butter in a large, heavy pan and sauté onions until tender. Stir in flour and cook for 2 minutes, stirring, without browning. Whisk in the stock. Add the cauliflower and seasonings and cook for about 20 minutes or until cauliflower is tender. Puree. Beat egg yolk into the cream and whisk into the hot soup. Do not allow to boil. Serve garnished with fresh parsley. Serves 8.