In a large heavy pot, melt butter; add onion and leeks and cook until soft but not brown. Add potato, carrots and squash; cook and stir 2 or 3 minutes. Add stock; cover and simmer about 20 minutes or until vegetables are tender. Puree in food processor or blender until very smooth. Return to saucepan. (Can be made ahead and frozen to this point). Stir in cream to desired consistency. Add wine if desired. Heat slowly until very hot (do not boil). Taste and season with salt and pepper. Serve garnished with a sprinkle of chives or green onions and a little grated carrot. Makes about 6 servings.