Source: Orangette by Molly Wizenberg
In a large saucepan over medium heat, warm 2 Tablespoons olive oil. Add the leek, onion, celery, and garlic, and sauté until softened but not browned, stirring occasionally, about 5 minutes. Add the celery root, broth, and salt, and bring to a boil. Reduce the heat to low, partially cover, and simmer until the celery root is very tender. It should break apart easily when poked with a fork (about 35-45 minutes). Remove the soup from the heat.
Using a blender purée the soup until very smooth (do this in batches- never fill a blender more then 1/3 full). Add the remaining ½ tablespoon oil and the milk, and stir to incorporate. Taste, and adjust seasoning as necessary. Reheat gently until just steaming.