While potatoes are cooking make a roux over low to medium heat. Mix butter, margarine, or light olive oil, and flour. DO NOT BURN THE ROUX. When roux is thickened a bit, gradually blend in milk. Continue cooking over low to medium heat while preparing potatoes.
Peel and cut up potatoes. You may want to mash some of the potatoes also. Add potatoes to the milk mixture. Blend in sour cream. Soup is ready to be served.