Prepare the butternut squash by peeling, seeding, and dicing it into 1-inch pieces. Chop the onion and mince the garlic.

In a large pot or Dutch oven, heat the olive oil over medium heat. Add the chopped onion and cook until softened, then stir in the minced garlic and cook until fragrant.

Add the diced butternut squash and vegetable broth to the pot. Bring to a boil, then reduce heat, cover, and simmer until the squash is very tender.

Carefully transfer the hot soup to a blender and blend until smooth, or use an immersion blender directly in the pot.

Return the soup to the pot if using a standard blender. Stir in the salt, pepper, and optional heavy cream. Heat gently until warmed through, then serve immediately.