roux
Also known as: white roux
Equal parts fat and flour cooked together into a paste, used to thicken sauces and soups. A restaurant kitchen often makes a batch ahead and keeps it on hand as its own measurable ingredient, rather than building it fresh into each dish -- see the roux technique entry for how it's made and how its color changes its thickening power and flavor.
Buying guidance
Not typically sold ready-made -- it's simple and quick enough that it's almost always made fresh from butter (or another fat) and flour. A "white" roux means cooked only briefly, just past raw-flour taste, with the least color and the strongest thickening power per ounce.
Substitutions
- A cornstarch or flour slurry stirred directly into the liquid -- faster, but gives a glossier, less rounded thickness than a properly cooked roux.
- Heavy cream, reduced -- adds body and richness without a starch thickener, though it won't thicken as firmly.
- A different color roux (blond or brown) -- less thickening power the darker it's cooked, but more toasted flavor; adjust the amount used accordingly.
Tips & common mistakes
Measuring roux by volume rather than weight is unreliable, since how much air is worked into it varies -- weighing it gives a more consistent thickening result batch to batch.
Storage
Keeps refrigerated for a couple of weeks in a sealed container, or frozen for months -- portioning it before freezing makes it easy to pull off just the amount a recipe needs.