Yes, you can cook butternut squash in a slow cooker. Add the onion, garlic, ginger (I still recommend sautéing these three first, if you can, but it's not necessary), cubed squash, chopped carrot, broth, and half the thyme to your slow cooker. Stir and add the lid. Set the slow cooker to low and cook for 4-6 hours, or set it to high and cook for 2-3 hours. You can then follow the instructions for puréeing the soup.