1. In a large saucepan, melt the butter. Add the onions and cook for 2 minutes over low heat.
2. Add the celery, celery root, potato, and garlic. Cover and let the vegetables sweat for 5 minutes.
3. Add the broth and season with salt and pepper. Cover halfway and continue cooking for 30 minutes.
4. Remove the saucepan from the heat. Using an immersion blender, reduce the soup to a fine purée and serve hot.