Preheat oven to 425°F and line a baking sheet with parchment paper.
Halve the pumpkin and scoop out the seeds. Remove any flesh from the seeds and discard but save the seeds for roasting, if desired.
Rub 1 tablespoon olive oil over the flesh of the pumpkin and season all over with salt. Place the halves cut side down on the prepared baking sheet and roast until knife tender, 35-40 minutes. Set aside to cool.
Heat the remaining 1 tablespoon olive oil in a large heavy-bottomed pot over medium heat. Add onion, carrots, celery, and garlic to the pot. Stir to combine and season with salt and pepper. Cook, stirring occasionally, until onion is translucent and vegetables are softened, about 10 to 12 minutes.
Once the pumpkin is cool enough to handle, scoop out the flesh and discard the skin.
Add the pumpkin flesh, sage, cinnamon, cayenne pepper, and black pepper and mix to combine. Pour in the broth. Bring the mixture to a boil, then reduce heat to a gentle simmer. Cook until flavors have blended and all vegetables are extremely soft, 15 minutes.
Stir in the maple syrup and apple cider vinegar and season to taste with salt and pepper. Use an immersion blender to blend the soup directly in the pot, or transfer the soup to a high speed blender and work in batches to purée.
Serve the puréed soup with toasted pumpkin seeds and fresh sage, if desired.
Storage: The cooled soup will keep well in the fridge for up to 5 days and in the freezer for 6 months.