Heat olive oil in a large stockpot over medium heat.
Add the diced onion and sauté for 5 to 7 minutes until translucent and softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the cubed butternut squash and vegetable broth to the pot; ensure squash is mostly submerged.
Bring the liquid to a boil, then reduce heat to low and simmer, covered, for 20 to 25 minutes or until the squash is fork-tender.
Remove the pot from heat and use an immersion blender to puree the mixture until completely smooth.
Stir in the heavy cream, salt, black pepper, and nutmeg.
Return the pot to low heat and simmer for an additional 2 to 3 minutes to incorporate flavors.
Ladle into bowls and garnish with chopped sage if desired.