Preheat the oven to 400°F and line a large baking sheet with parchment paper.
Brush the cut sides of the squash with 1 tablespoon olive oil, place face down on the prepared sheet, and roast for about 45 minutes or until caramelized and tender.
While the squash roasts, heat the remaining 1 tablespoon olive oil in a large skillet over medium heat.
Add the onion and cook until softened, about 5 minutes.
Stir in the garlic, salt, maple syrup, cinnamon, and black pepper, cooking for another 1–2 minutes until fragrant.
Transfer the onion mixture to a large blender.
When the squash is done, allow it to cool slightly, then scoop the flesh into the blender with the onions.
Add 4 cups of broth to start, and let the mixture cool before blending until smooth. Add additional broth to reach your desired consistency.
Pour the blended soup into a large pot and stir in the butter.
Warm gently over medium-low heat until heated through, then taste and adjust seasoning before serving.