The French name for the flavour base made from diced onion, carrot, and celery is mirepoix.
Mirepoix is an aromatic vegetable mixture used to build depth in stocks, soups, sauces, stews, and braises. The classic proportion is two parts onion to one part carrot and one part celery, although recipes vary. The vegetables are usually diced and cooked slowly in butter, oil, or another fat so they soften and sweeten without necessarily becoming deeply browned.
The term dates from 18th-century French cuisine and is linked to Charles-Pierre-Gaston de Lévis, the Duke of Mirepoix, whose household cook is traditionally credited with popularizing the name. The technique itself is older, and historical versions sometimes included ham or other meat alongside the vegetables.
Mirepoix is often confused with Italian soffritto, which commonly uses the same three vegetables but is generally cooked more actively and may brown. Duxelles is different: it is a finely chopped mushroom preparation, while roux is a cooked mixture of fat and flour used to thicken sauces.