This soul-satisfying soup has all the best Fall flavors and makes a lovely starter on the holiday table. It freezes well and makes a great not-sad desk lunch the day after you make it. This recipe makes 6 servings.
Preheat your oven to 425 F. Cut the squash into quarters; discard seeds. Place on a baking sheet, and drizzle with 2 tablespoons of the olive oil. Bake in preheated oven until fork-tender, about 45 minutes to 1 hour. Cool until the squash can be handled. Remove skin, and discard. Set aside 1 1/2 cups (about 15 ounces) cooked pumpkin; reserve remaining pumpkin for another use.
Heat remaining 2 tablespoons olive oil in a large saucepan over medium. Add onion, celery, carrots, garlic, thyme and sage; cook, stirring occasionally, until vegetables are very tender, 8 to 10 minutes. Add salt and turmeric; cook for one more minute. Stir in chicken broth and 1 1/2 cups cooked pumpkin; bring to a simmer. Cover and simmer 15 minutes. Remove and discard herb sprigs.
Place half of pumpkin mixture in a blender. Remove center piece of blender lid to allow steam to escape; secure lid on blender, and place a clean towel over opening in lid. Process until mixture is very smooth, 1 to 2 minutes. Transfer to a large bowl. Repeat with remaining pumpkin mixture. Stir in the apple cider vinegar.
Whisk together sour cream and milk in a small bowl. Divide soup evenly among 6 shallow bowls; drizzle with sour cream mixture, and sprinkle with pumpkin seeds, if desired. Serve immediately.