Heat the olive oil in a large saucepan over low heat then sauté the onion for about 5 minutes.
Remove the outer skin from the celery root and cut the inner root into small chunky pieces.
Stir the celery root chunks into the pan with the onions then add in the water and bouillon; bring to a boil.
Cover, reduce the heat and simmer for about 20 minutes or until the celery root is tender.
Add in the parsley, salt, pepper, and lemon juice then use the immersion blender to puree your soup. Serve while still hot and enjoy.