In a large pot, saute the onion, carrots and celery until soft - about 5-7 minutes. Add the chicken and saute until chicken is fully cooked through.
Transfer the chicken and vegetables to a bowl and set aside.
Add broth and celery root to the pot and cook for 12-15 minutes. The celery root should be tender when pierced with a fork.
Using an immersion blender, puree the celery root and broth until smooth and creamy.
Add the chicken and vegetable mixture, thyme, parsley, sage and salt to the pot. Simmer on medium heat for 8-10 minutes until fragrant. Remove from heat and serve immediately. Enjoy!