Cut the broccoli into florets and set them aside. Peel the tough outer layer from the broccoli stalk, if needed, and cut the stalk into small pieces.
Add the celery root and chopped broccoli stalk to a large soup pot along with the vegetable broth.
Bring to a boil, then reduce the heat and simmer for about 15 minutes, or until the vegetables are almost tender.
Add the broccoli florets and cook for another 3–5 minutes, until tender.
Use an immersion blender to blend the soup until smooth. If the soup is too thick, add a little extra water or broth until you reach your desired consistency.
Make the Cheese Croutons
Cut the bread into small cubes.
Place the bread cubes in a bowl and toss with the olive oil and grated Parmesan cheese.
Heat a skillet over medium heat and cook the bread cubes, turning frequently, until golden brown and crispy.
Ladle the soup into bowls and top with the cheese croutons just before serving.
Notes
For a creamier soup, add a splash of heavy cream or coconut cream after blending. If you prefer a stronger flavor, roast the broccoli and celery root before adding them to the broth.