Juice the lemon into a large bowl of water. Peel the Jerusalem artichokes. Chop them into small chunks and drop them into the water as you go (this helps stop them turning brown). Set aside.
Trim the roots and tips from the leek. Slice a cross 3cm into the top and open it out to rinse out any grit. Finely slice the leek. Trim the dry ends off the celery and finely slice it.
Warm a large pan over a medium heat for 1 min. Add 1 tbsp oil, the leek and celery. Season with salt and pepper. Cook for 8 mins, stirring often, till the veg have softened.
Drain the Jerusalem artichokes and add them to the pan. Add the bay leaf and crumble in the stock cube. Pour in 800ml boiling water. Cover, bring to the boil and simmer for 20 mins till the artichokes are soft. Lift out the bay leaf.
While the soup simmers, peel, core and chop the apples. Add them to the pan. Simmer for 5 mins. Ladle them into a blender and blitz till smooth, or use a hand-held blender in the pan.
Taste the soup and add more salt or pepper if you think it needs it. Finely chop the chives and swirl them into the soup. Ladle into warm bowls and serve.