Trim cauliflower and rinse well under cold water. Cover the cauliflower with water and steam until very tender. While cauliflower is steaming, sauté onion in the butter until softened. Add flour to butter and onion and cook for a few minutes. Stir the flour to blend well into butter. Slowly add 3 cups of milk, the yogurt, and stir or whisk until thickened. Drain cauliflower and blend in a food processor until smooth. If desired, process the milk mixture for a smoother texture. Add pureed cauliflower to thickened milk mixture and stir well. Heat through. Garnish with a little chopped parsley and serve with Parmesan cheese. Makes 4 servings. Note: this is an excellent back to school dinner recipe that can be doubled and re-heated the next day for lunches.
