Add all the ingredients (except for the coconut milk, if using) to a slow cooker and cook on high for 3-4 hours or low for 4-5 hours until the squash is soft and tender.
Once it's cooked, either blend directly in the slow cooker using an immersion blender or carefully transfer to a blender and mix until smooth and creamy. Blend in the coconut milk now, if using.
Season with salt and pepper if needed and serve.
