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. Stir them every 5 to 7 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.
Remove the pot from the burner, deglaze with the bourbon, and cook until the liquid has been absorbed, which takes about 2 to 3 minutes.
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.
Season with salt and ground white pepper and cook on low heat until ready to serve.
Whip the remaining 1 cup of heavy whipping cream in a stand mixer using the whisk attachment until stiff peaks are formed, fold in maple syrup, and chill until ready to serve.
Serve the soup with two tablespoons of maple whipped cream and add optional garnishes of fresh sage leaf and pepitas.
Notes
Make-Ahead: You can make this up to 2 days ahead for freshness.How to Store: Cover the soup in the refrigerator for up to 5 days. Keep it separate from the maple whipped cream garnish. Cover the soup and place in the freezer for up to 2 months. Thaw in the refrigerator for 1 day before reheating.How to Reheat: Place the desired amount into a medium-size pot and cook over low heat until hot.If using a blender, take the center part to the top off because the soup will be hot, and pressure will build, causing it to explode everywhere if kept on.