In a slow cooker, combine seasoning packet from au gratin potato mix, chicken broth, and water. Whisk to dissolve seasoning.
Add potatoes from au gratin mix, carrots, celery, onion, salt, and pepper.
Cover and cook on low for 2-4 hours, until potatoes are tender.
Stir in whipping cream, cover, and cook for 30 more minutes.
Using an immersion blender, carefully blend soup to desired consistency.