Clean the squash by removing the ends and cutting down the middle lengthwise. Scoop out the seeds with a spoon and peel and dice the squash into 1-inch pieces. Place in a 4- or 6-quart slow cooker. Add the celery, carrots, onion, smashed garlic cloves, apple, fresh ginger, curry powder, cinnamon, kosher salt, and black pepper.
Pour in the chicken stock, cover with the lid, and set on high for 6-8 hours. When finished, a knife should pierce the squash and apples with no resistance.
Add the apple cider vinegar and coconut milk.
Use an immersion blender and purée the soup, leaving a little texture. Alternatively, the soup can be puréed in a blender in batches. If using a countertop blender, make sure to leave the vent slightly ajar to allow any steam to escape.
