Preheat oven to 400°F (200°C). Cut the butternut squash in half lengthwise and scoop out the seeds. Brush the cut sides with olive oil and place cut-side down on a baking sheet.

Roast the squash for 35-40 minutes until very tender when pierced with a fork. Remove from oven and let cool slightly.

While squash cools, heat 1 tbsp olive oil in a large pot over medium heat. Add chopped onion and sauté until translucent, about 5 minutes. Add garlic and cook 1 minute more.

Scoop the roasted squash flesh from the skin and add to the pot along with vegetable broth, cinnamon, and nutmeg. Bring to a simmer.

Using an immersion blender, puree the soup until completely smooth. Alternatively, carefully transfer to a blender in batches.

Stir in heavy cream and season with salt and pepper to taste. Simmer for 5 more minutes to allow flavors to blend.

Ladle into bowls and garnish with a drizzle of cream and pumpkin seeds if desired. Serve warm.