Flour is added to French onion soup primarily to give it body and depth.
Here's a breakdown:
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Thickening: The flour acts as a thickening agent. When cooked with the onions, it absorbs some of the rendered fat and moisture, creating a roux. This roux then helps to slightly thicken the soup, giving it a more substantial texture than just broth.
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Enhancing Flavor: The slight nuttiness that comes from toasting the flour (as it cooks with the onions) adds another layer of flavor complexity to the soup. This subtle flavor enhancement contributes to the overall rich taste profile.
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Creating Body: Without a thickening agent, French onion soup can be quite thin. The flour gives the soup more body, making it feel more satisfying and hearty.
While some recipes omit flour, those that include it generally do so for these key reasons. It's important to note that the amount of flour used is typically small, so it doesn't drastically change the flavor or texture, but rather subtly enhances the overall quality of the soup.
