Trim cauliflower; cover with water and steam until very tender. While cauliflower is steaming, sauté onion and the green onions in the olive oil until softened. Add the cauliflower and stir well. Slowly add 3 cups of milk, mashed potato, the sour cream or yogurt, and stir or whisk until mixed well. Add the mixture to a food processor and blend until smooth. Heat through. Garnish with a little chopped parsley and serve with Parmesan cheese. Makes 4 servings.