In a saucepan, sauté the onion in the butter, then add chicken broth and squash.

Cover and simmer until the squash is tender.

Puree the squash in a blender or food processor, adding the buttermilk for more liquid, if needed.

Return the mixture to the saucepan and add the remaining buttermilk, adjust seasonings to taste.

Bring to a boil and serve immediately.