Mirepoix is a recipe base made from diced vegetables that are cooked slowly (usually with some sort of fat, like butter or oil) to sweeten and deepen the flavors of a dish. The mix is cooked over very low heat, as the intention is to intensify the vegetables’ flavor — not to caramelize them.
French mirepoix is made with celery, onions, and carrots. The traditional ratio is 1:2:1 (one part celery, two parts onions, and one part carrots). The trio serves as the foundation for many Western soups, stews, and sauces. The coarseness of the vegetables is dependent on the mother recipe.
A close relative is the Cajun Holy Trinity made from equal parts celery, onions, and bell peppers.