Add the butter to a large rondeau or Dutch oven pot over low to medium heat and add the diced onions until they are caramelized, which takes about 35 to 40 minutes.
Add garlic and sauté until fragrant, which takes 35 to 40 seconds. Pour the prepared butternut squash and lightly cook on low to medium heat for 10 to 15 minutes to soften it.
Pour the stock and simmer over low heat for 20 to 25 minutes or until the squash is tender. Add the 1 cup of cream, nutmeg, cinnamon, and puree using an emersion or blender until smooth.