2. Cut the butternut squash in half, scrape out the seeds and set them aside. Place the squash, cut side down, on a baking sheet and bake until tender and easily pierced with a fork, about 40 minutes. Let cool about 10 minutes.
3. While the squash is baking, heat the oil in a small skillet over medium heat. Add the onion and celery, and cook until soft and translucent, about 3 minutes.
4. Scoop the squash pulp out of the skin with a spoon and put it into a large bowl. Add the onion, celery and milk or stock. Stir to combine.
5. Puree this mixture in a blender in batches, being careful not to overfill the blender. Strain the blended mixture through a colander to remove any remaining fiber or seeds.
6. Add the lemon juice, cumin, allspice and sherry or Marsala wine. Season with salt and pepper to taste.
7. Spread the cleaned, reserved seeds on a baking sheet and toast in the oven until lightly browned, about 10 minutes.
8. Reheat the soup, adding milk or stock if it is too thick. Divide it among 6 warmed soup bowls and sprinkle with the toasted seeds.