Heat oil in a large saucepan over medium heat. Add leeks and cook, stirring, for about 5 minutes.
Add celeriac and apples (if using) and cook, stirring, for another 5 minutes.
Add water and bring to a simmer, then cover and cook until celeriac is tender, about 30 minutes.
Puree soup mixture until smooth in an upright blender, food processor or directly in the saucepan with an immersion blender.
Stir in dried thyme leaves, pepper to taste and additional salt if necessary.
Serve soup with a garnish of fresh thyme leaves.
