Peel and finely chop the onion. Trim and finely chop the celery. Warm a large pan over a low heat for 1 min. Add 1 tbsp olive oil, the onion and celery. Season with salt and pepper. Cover and sweat for 10 mins.
Slice the roots off the celeriac at a diagonal, then peel the rest of the celeriac. Roughly chop it. Peel, core and chop the apple. Stir both of them into the pan. Add 1 bay leaf and half the sage leaves.
Crumble in a stock cube, then pour in 850ml boiling water (or use hot homemade stock). Cover, turn the heat up and bring to the boil. Turn the heat down a little and simmer for 20 mins till the veg are very tender.
Fish the bay and sage leaves out of the soup. Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Taste and add more salt and pepper if you think it need sit. Serve garnished with a few sage leaves.