What is a common soup thickening method that uses a mixture of flour and fat?

The correct answer is the method known as roux, which involves combining equal parts of flour and fat, typically butter, and cooking this mixture until it reaches a desired color and flavor profile. This technique serves as a foundational element in many classic sauces and soups, acting as a thickening agent when blended into liquids.

Roux is usually cooked to different stages, from white to brown, which contributes not only to the thickness but also to the depth of flavor in the finished dish. For example, a light roux is used for cream soups and sauces, while a darker roux is preferred for dishes like gumbo, as it imparts a rich, nutty taste.

In contrast, beurre manié is a mixture of flour and uncooked butter that is kneaded together and added at the end of cooking to thicken sauces or soups, which is different from the roux method that requires cooking the flour and fat together first. A slurry is a mix of cornstarch and water, used for thickening but does not include fat. Gelling refers to the process of creating a gel-like consistency, which is not a traditional thickening method involving flour and fat.