Description
The mirepoix is a fancy French name for the aromatic vegetables at the base of a great many sauces and braises. It's simply 2 parts by weight of onion to one part each of carrots and celery.
You use a mirepoix when making any brown sauce (Escoffier's Sauce a l'Espagnole and its many fine children), many red sauces, and most white meat demiglazes. You also use it when braising meat, as in a Pot Roast, and in many stews.