Follow the steps in order for the best result.
Begin by preheating your oven to 350°F (175°C).
Lightly coat the cut sides of the halved garlic heads with olive oil. Reassemble the heads by pressing the halves back together.
Wrap each garlic head tightly in aluminum foil and bake until the garlic is soft and tender, approximately 40 minutes.
Allow the roasted garlic to cool inside the foil.
In a large, heavy-bottomed pot, melt the butter over medium heat.
Add the chopped onions, carrots, and celery to the pot and sauté until the onions begin to soften, about 5 minutes.
Incorporate the cubed butternut squash, chicken broth, and 2 tablespoons of the chopped fresh sage.
Bring the mixture to a boil, then reduce the heat to low. Simmer uncovered until the squash is completely tender, around 25 minutes.
While the soup simmers, unwrap the roasted garlic. Squeeze the softened garlic cloves from their skins into a small bowl. Discard the skins.
Mash the roasted garlic with a fork until it forms a smooth paste.
Stir the roasted garlic paste into the simmering soup.
Working in batches, carefully purée the soup in a blender until it reaches a smooth consistency.
Return the puréed soup to the pot and gently reheat. At this stage, the recipe can be prepared a day in advance. Refrigerate uncovered until cold, then cover and keep refrigerated. Bring to a simmer before proceeding.
Stir in 1/2 cup of whipping cream and season the soup to your liking with salt and pepper.
Transfer the butternut squash bisque to a tureen or serving bowl.
Drizzle the remaining 1 tablespoon of cream over the top and sprinkle with the remaining 1 tablespoon of chopped fresh sage before serving.