Heat the coconut oil over medium-high and add the chopped onion. Saute, stirring occasionally, until onion begins turning golden brown, about 8 to 12 minutes. Add the garlic and continue cooking another 2 minutes. Transfer the onion and garlic to your slow cooker.
Add the remaining ingredients for the soup to the slow cooker. Cook on low for 6 to 8 hours (or on high for 4 to 5 hours), or until butternut squash is soft and cooked through.
Use an immersion blender to blend the soup to your desired consistency. If you don't own an immersion blender, allow the soup to cool significantly before transferring it to a high-powered blender such as a Vitamix or a BlendTec. This will need to be done in batches, as the volume of the soup is too large for one batch. Serve with a drizzle of olive oil (or coconut milk), chopped pistachios and pecans.