In a large pot, heat the olive oil and add the onion and celery. Cook on medium-high heat for about 5 minutes, stirring often. Add the garlic and heat over medium-low heat for an additional 5 minutes. Season with salt and pepper.
Add 3 cups of chicken broth, apple, butternut squash, and spices. Bring to a boil over high heat. Once boiling, cover and reduce the heat. Simmer for about 30 minutes or until the squash is tender.
Using an immersion blender, blend the soup until smooth. For a traditional blender, be very careful transferring the soup. It will need to be done in batches.
Once completely smooth, add the heavy cream and season with additional salt and pepper, if necessary. If the soup still seems too thick, add the remaining chicken broth. Serve hot with sour cream, green onion, pepitas, and crusty sourdough bread.