In a large saucepan, heat butter over medium. Add onion, garlic, thyme, cinnamon, and cayenne. Season with salt, and cook, stirring occasionally, until onion is softened, 5 to 7 minutes.Advertisement
Add squash, broth, half-and-half, and 3 cups water. Bring to a boil; reduce to a simmer, and cook until squash is tender, about 20 minutes.
Working in batches, puree in a blender until smooth. Stir in lemon juice; season with salt. Serve bisque with sour cream, garnished with cayenne, if desired.
For a different flavor, use ground nutmeg or cloves instead of cinnamon, or add a Southwestern touch with a small amount of chili powder. Ladle cooled bisque (without sour cream) into airtight containers, leaving 1 inch of space; freeze up to 3 months.