In a medium saucepan, heat olive oil over med-high heat. Add the scallions and cook for 2 - 3 minutes until softened. Add in the Celery Root, Celery, Apple, Potatoes, Garlic, Salt, and Pepper and stir to coat. Add in the water and broth and bring to a boil. Lower heat to simmer and cover. Cook for 25 minutes until vegetables are tender. Turn off heat.