Start with butter, chopped onions and celery. After sauteing the onions and celery for about 5 minutes, I then add the flour to make a roux.
In this soup, the roux is used as a thickener and a way to make the soup smooth and velvety.
Let the roux mixture cook for 5-7 minutes over low heat. Add the stock to the roux, using a wire whip, and mix well.
If you have an immersion blender, use your it to puree as much of the product as possible.