1 cup thinly sliced leek (about 1 medium), white and light green parts only
2 1/2 pounds celery root, also known as celeriac (about 3 medium), peeled and cut into 1-inch chunks
12 ounces Yukon Gold potatoes (about 2 large), peeled and cut into 1-inch chunks
1 medium tart apple, such as Granny Smith, peeled, cored, and cut into 1-inch chunks
2 medium garlic cloves, peeled and smashed
2 teaspoons kosher salt, plus more as needed
Freshly ground black pepper
3 cups water
2 cups low-sodium chicken or vegetable broth
Heat oil in a large saucepan with a tightfitting lid over medium-high heat until shimmering. Add leek and cook, stirring occasionally, until softened and translucent, about 3 minutes. Add celery root, potatoes, apple, garlic, salt, and a pinch of pepper. Stir to coat vegetables with oil, add water and broth, and bring to a boil. Cover, reduce heat to low, and simmer until vegetables just give way when pierced with a knife, about 20 to 25 minutes.
Remove 1 cup of liquid from the saucepan; set aside. Using a blender, purée the soup in batches until smooth, removing the small cap from the blender lid (the pour lid) and covering the space with a kitchen towel (this allows steam from the hot soup to escape and prevents the blender lid from popping off). Once blended, transfer the soup back to the saucepan and keep warm over low heat. If the soup is too thick, add the reserved liquid a little at a time until the soup reaches the desired consistency. Taste and season with additional salt and pepper as needed. To serve, drizzle with olive oil.