In a soup kettle or Dutch oven, saute the onions, carrots, celery and garlic in 1/4 cup butter until tender. Add cauliflower, broth, parsley, salt and pepper, basil and tarragon. Cover and simmer for 30 minutes or until the vegetables are tender.
Meanwhile, in a saucepan, melt the remaining butter. Stir in flour until smooth. Gradually stir in the milk and whipping cream. Bring to a boil; cook and stir for 2 minutes or until thickened. Add to cauliflower mixture. Cook for 10 minutes or until thickened, stirring frequently. Remove from the heat; stir in sour cream. Garnish with tarragon if desired.Yield: 8 servings (about 2 quarts).