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.