Place the onion, potato, celery root, celery, bay leaf, thyme, and broth in a large 4-quart pot. Set over medium heat and bring to a simmer. Reduce heat to low to maintain a simmer for 20 minutes. After 20 minutes, use a pairing knife to pierce the celery, celery root, and potatoes to see if it breaks apart when pierced. If your vegetables are softened remove the bay leaf and parsley.