A roux is a mixture of equal parts flour and fat, which can be butter, oil, or a combination of both. They are cooked together to make a paste that is the base for creamy sauces and soups. By cooking the flour, you remove the raw flavor and create a delicious, rich thickener. Then when liquid is added, it incorporates smoothly without clumps and creates a velvety finish to your creamy and smooth soup.