In a large stockpot, saute diced onion in butter until translucent.

Peel, seed, and chop butternut squash into 1-inch pieces. Add butternut squash and chicken broth to onion in the stockpot.

Bring the mixture to a boil, then cover and simmer for 15-20 minutes (until squash is tender).

Using an immersion blender, or working in batches with a regular blender, puree soup to desired consistency.

Season to taste with salt, pepper, nutmeg, and cinnamon.