Step 1

Heat oil in a large saucepan over medium heat. Add leeks and cook, stirring, for about 5 minutes.

Step 2

Add celeriac and apples (if using) and cook, stirring, for another 5 minutes.

Step 3

Add water and bring to a simmer, then cover and cook until celeriac is tender, about 30 minutes.

Step 4

Puree soup mixture until smooth in an upright blender, food processor or directly in the saucepan with an immersion blender.

Step 5

Stir in dried thyme leaves, pepper to taste and additional salt if necessary.

Step 6

Serve soup with a garnish of fresh thyme leaves.