Preheat the oven to 400°F (200°C). Cut the squash in half, remove seeds, and place cut-side down on a baking sheet lined with parchment. Roast until tender and slightly caramelized, about 30-40 minutes.
While the squash roasts, heat a tablespoon of olive oil in a large pot over medium heat. Add the chopped onion and sauté until translucent and lightly golden, about 5-7 minutes, releasing a sweet aroma.
Remove the roasted squash from the oven and let it cool slightly. Scoop out the flesh and add it to the pot with the sautéed onions.
Add the chopped sage and ground nutmeg to the pot, stirring to coat the vegetables and release their fragrant aroma.
Pour in the vegetable stock and bring the mixture to a gentle simmer. Cook uncovered for about 10 minutes to allow flavors to meld.
Use an immersion blender or transfer the mixture to a blender in batches, blending until smooth but still with some texture, about 1-2 minutes.
Return the pureed soup to the pot if using a blender, and gently reheat over low heat, seasoning with salt and pepper to taste.
Taste and adjust seasoning as needed, making sure the flavors are balanced and fragrant.
Serve the soup hot in bowls, garnished with fresh herbs or a drizzle of olive oil if desired, showcasing its vibrant color and velvety texture.
