What is a roux?….a roux is commonly used as the thickening base for soups, sauces, and gravies.  A simple white roux like the one used in the Cream of Potato – Leek soup  is a combination of fat and flour cooked together over low heat for about 5 minutes.  It is then added to a liquid – in this case milk, brought to a low boil for maximum thickening and removed from the heat.  Liquid ingredients other than milk (like broth or a combination of broth and milk) can comprise a white sauce.  But a simple white sauce recipe follows:

2   tablespoons of butter melted
2   tablespoons of  flour
1   cup of milk

Combined the butter and flour together and cook for 3- 5 minutes over low heat.  Slowly whisk in the milk, bring it to a very low boil, stirring constantly and take it off the heat once thickened.  Add seasonings.
For a very thick white sauce use:  3 tablespoons butter, 3 tablespoons flour, and one cup of milk and follow the same procedure.