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, sauté 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 liquid until
soup is desired consistency.
Yield: 8-10 servings (about 2 ½ quarts).