Melt the butter in a saucepan. Add the garlic, onion, carrot and celery and cook on low until tender and translucent. Add chicken stock and the sweet potatoes, increase the heat a little and cook, stirring from time to time, until the sweet potatoes are tender. Add the apples and cook another 5 minutes or so, until they are tender.
Add heavy whipping cream, season with salt and pepper. Allow soup to cool slightly, then purée it with an immersion blender or in a blender. You may have to divide this in two batches.