Don't get flustered by the fancy French name!
Among all the classic French techniques, making a roux is not only the simplest, but also the most useful. It forms the base for everything from a creamy mac n' cheese to Cajun gumbo.
Pronounced "roo" as in kangaroo, a roux is equal parts butter and flour, mixed together, and then cooked briefly over medium heat.
Broth or milk is then whisked in, the starch molecules in the flour absorb the liquid, and the whole mixture thickens into a sauce.
The exact amount of butter and flour varies: the more roux you make, the thicker your sauce. For a thin sauce or soup base, use one or two tablespoons of each for every one cup of liquid you're planning to add. For a thicker base you can bump it up to three tablespoons.
Melt the butter first and then add the flour. It will be clumpy at first, but keep stirring it into a smooth, thick paste.
You only need to cook the roux for a minute or two--you're not actually going for color. Pour the liquid in slowly, whisking constantly to avoid clumps and so the liquid is absorbed evenly.
If you're adding veggies, saute them in the butter before adding the flour.
Keep in mind that the longer a roux is cooked, the more it loses its ability to thicken.
Recipes that call for long-cooked brown roux, like gumbo and espagnol sauce, are using it as more of a flavor base and will usually have you add another kind of thickener toward the end of cooking.
Not so bad, right?! Next time you come across a recipe with "make a roux" in the instructions, you'll know it's nothing to be scared of!