In a Dutch oven, cook the potatoes, carrots and celery in water until tender, 15-20 minutes. Drain, reserving liquid and setting vegetables aside. , In the same pan, saute onion in butter until tender. Stir in the flour, salt and pepper; gradually add milk. Bring to a boil, cook and stir for 2 minutes or until thickened. Gently stir in cooked vegetables. Add 1 cup or more of reserved cooking liquid until soup is desired consistency.