Preheat oven to 400°F (200°C). Peel, deseed, and chop the butternut squash into 1-inch cubes. Toss with 1 tbsp olive oil, salt, and pepper, then roast for 25-30 minutes until tender.

Heat the remaining 1 tbsp olive oil in a large pot over medium heat. Sauté chopped onion until softened, then stir in minced garlic and cook for another minute.

Add the roasted butternut squash and vegetable broth to the pot. Bring to a simmer, then reduce heat and cook for 10-15 minutes to meld flavors.

Carefully purée the hot soup in a blender or with an immersion blender until smooth. If desired, stir in heavy cream at this stage.

Adjust seasoning, then ladle into bowls and garnish with fresh sage before serving.