Mirepoix
A French base of diced onion, carrot and celery cooked gently in fat.
Mirepoix is the standard aromatic base of French cooking. Onion, carrot and celery are diced, commonly two parts onion to one part each of the others, and cooked slowly in butter or oil without browning. The mixture gives a background sweetness to stocks, soups, stews and sauces, and is often strained out at the end. Other cuisines have their own trios. Cajun and Creole cooks use onion, celery and green pepper.