Preheat oven to 375°. Cut squash or pumpkin in half and scoop out seeds. Spread a thin film of olive oil on a baking sheet and place the pumpkin halves cut side down. Roast until tender, about 45 minutes to an hour (or more), depending on the thickness of the flesh. Remove from oven and let cool. Scoop out the flesh and set aside.
Heat the butter in a large saucepan over medium heat. Add the onion and celery and cook, stirring often, until softened (about 5 minutes.) Add the sage and cook a few minutes more.
Drain any excess liquid from your squash puree. Add the puree and stock to the pan. Turn up the heat and bring to a boil. Reduce heat and simmer, partially covered, for about 20 minutes.
Puree with an immersion blender until completely smooth, or blend in batches in a blender and return to the pan.
Add the maple syrup, salt, and heavy cream and blend well.
Taste and adjust the seasonings. Serve garnished with pepitas.