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.
While the veg sweat, slice the roots off the celeriac on the 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 most of the rosemary sprigs.
Crumble in the stock cube, then pour in 850ml boiling water. 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 woody rosemary sprigs out of the soup (don’t worry if a few leaves are left behind). 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 needs it. Serve garnished with a few fresh rosemary leaves.