How to Make It
In a large saucepan, heat the olive oil until shimmering. Add the shallots and a generous pinch of salt and cook over moderate heat, stirring, until the shallots are softened and just starting to brown, about 5 minutes. Stir in the celeriac and celery and cook until the celery just starts to soften, about 5 minutes. Add the stock and lemon half, cover and simmer over moderately low heat until the celeriac is tender, about 30 minutes.
Remove and discard the lemon half. Working in batches, puree the soup in a blender. Return the soup to the saucepan and warm over low heat. Stir in the lemon juice and season with salt. Ladle the soup into bowls, garnish with celery leaves and serve.