Layer sliced potatoes in a lightly greased 6-quart slow cooker; top with chopped onion.
Stir together chicken broth, salt, and pepper; pour over potatoes and onion. (Broth will not completely cover potatoes and onion.) Cover and cook on HIGH 3 to 5 hours or until potatoes are tender. Mash mixture with a potato masher; stir in half-and-half. Cover and cook on HIGH 20 more minutes or until mixture is thoroughly heated. Ladle into bowls, and serve with desired toppings.