2. Add the onion and celery and cook until tender and translucent, about 10 minutes. Add the shallots and thyme, season lightly with salt and pepper and cook a few minutes longer.
3. Meanwhile, peel and cube the celery root and add it to the pot along with the chicken broth. Bring to a boil. Reduce heat to a simmer and cook, covered, until celery root is tender, about 30 to 40 minutes.
4. Meanwhile, core and dice the apples and sprinkle them with lemon juice so that they do not discolor. Set aside for garnish.
5. Puree the soup in a blender.
6. Ladle the hot soup into bowls and serve garnished with diced apples and crumbled blue cheese.