Heat butter in a large heavy saucepan. Add the onion and garlic; sauté until tender, about 5 minutes. Add cauliflower and chicken broth. Bring to a boil; cover and simmer for 40 minutes.
Place the cauliflower mixture in a blender; add the milk and purée until smooth. Return to the pan and add the salt and pepper to taste. May be prepared in advance up to this point.
Reheat the soup prior to serving, but do not boil. Add the half-and-half during the last few minutes. Garnish with fresh parsley and croutons. Yield: 4 servings.