Mirepoix is a blend of vegetables used as the base for many French dishes. Mirepoix forms the base for soup stocks, sauteed meals, stews, and a wide variety of other foods, and it is sometimes referred to as the “holy trinity” of French cooking. Many cuisines have some sort of vegetable and spice blend similar to mirepoix, reflecting the fact that national cuisines are often built upon a small group of familiar flavors used in a wide variety of innovative ways.
In its purest form, mirepoix consists of onions, celery or celeriac, and carrots in a 2:1:1 ratio. The vegetables are diced or minced finely and as evenly as possible, and they are sauteed in butter in the pan before any other ingredients are added, creating a rich depth of flavor which will develop as the dish is cooked. Some cooks add herbs to their mirepoix, and it is also possible to see cubed meats, typically fatty cuts of meat which will brown and crisp in the pan.
According to legend, mirepoix is named for an 18th century chef who cooked for a French ambassador. However, documented use of this seasoning mixture is much older, and while Mirepoix the chef may have popularized it, he certainly wasn't the first to use it. Francophone cooks in the American South such as the Cajuns use a version of mirepoix made with celery, onions, and bell peppers instead of carrots for their own version of the holy trinity.
While these vegetable ingredients might sound simple, the browning in butter generates a Maillard Reaction, triggering a cascade of events. This reaction is responsible for the browning and tenderizing of the vegetables, but it also releases their natural sugars, allowing them to develop a complex caramelized flavor. A rich assortment of aromas and flavors develop as the vegetables are browned, and these flavors are incorporated into the finished dish. In addition to being used at the beginning, mirepoix may also be added into a dish at the end to refresh the flavors.
In French restaurants, mirepoix is a common item in the mise en place, the assortment of prepped food items which are placed near a chef's station. Mirepoix is used so much that preparing it ahead of time makes much more sense than having to freshly chop vegetables for each dish which requires it. Some French home cooks also keep mirepoix ready for use in the refrigerator, as the vegetables will keep for several days after chopping.