Add the cubed butternut squash, diced onion, and minced garlic to the slow cooker.
Pour in the vegetable broth, then stir in the dried sage, salt, and black pepper.
Cover and cook on low for 6 hours or until the squash is very soft.
Use an immersion blender to puree the soup directly in the slow cooker until smooth.
Stir in the heavy cream and let the bisque warm for 10 more minutes on low.
Ladle into bowls and top each serving with a few fresh sage leaves.