Preheat oven to 400°F. Place squash halves onto a baking sheet and drizzle with 1 tablespoon oil. On another baking sheet, arrange diced squash and onion. Drizzle with remaining 1 tablespoon of oil and sprinkle with sea salt and pepper to taste. Sprinkle with the sage.
Roast the squash halves and the diced squash and onion until fork-tender (roast the diced squash for 30 to 40 minutes and the squash halves for 40 to 50 minutes).
Set the roasted squash halves aside. Add the roasted diced squash and onion to a high speed blender. Blend on high, or until smooth and creamy. Pour the mixture into a large Dutch oven and stir in the vegetable broth and the butternut squash soup. Bring to boil, reduce to a simmer, and cook for 10 minutes. Serve the soup in the roasted squash halves. Sprinkle with additional fresh sage, if desired.
This nutritional information refers to each serving of the entire recipe and not just the products used as ingredients.