roux
n : a mixture of fat and flour heated and used as a basis for
sauces
Definition: A roux is a mixture of butter and flour, cooked until bubbly. It can be browned very deeply, then used as the basis for etouffe and brown sauce. If not browned at all, it is the base of bechamel, veloute, or white sauce. By cooking the flour in the butter, the starch granulesin the flour begin to break. Then when liquid is added, the granules absorb the liquid, thickening the sauce.
________
In classical cuisine, you use roux to make sauces. I use it whenever I thicken a sauce that is non-Asian in origin, or whenever I make a reduction. The only times I use cornstarch is when I do Chinese cooking, but hey, Marco, do whatever you like.