In a large kettle, cook potatoes, carrots and celery in water until tender, about 20 minutes. Drain, reserving liquid and setting vegetables aside. In the same kettle, saute onion in butter until soft. Stir in flour, salt and pepper; gradually add milk, stirring constantly until thickened. Gently stir in cooked vegetables. Add 1 cup or more of reserved cooking servings, about 2-1/2 quarts.