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Melt butter in a heavy large pot over medium-high heat. Add squash, onion, celery and carrot; sauté until slightly softened, about 15 minutes.
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Rinse and strip thyme and sage from stem. Add to pot along with apples.
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Add broth and 1 cup cider; bring to boil.
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Reduce heat to medium-low, cover, and simmer until apples are tender, stirring occasionally,
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Working in batches, purée soup in blender until smooth then return to pot. Season to taste with salt and pepper.
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Boil remaining 1/2 cup cider in heavy small saucepan until reduced to 1/2 cup, about 5 minutes.
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Cool.
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Place sour cream in a small bowl. Whisk in reduced cider.
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Mix whipping cream into soup.
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Lade soup into bowls. Drizzle with cider cream.