Mirepoix

Mirepoix is the French name for the classic culinary term for a combination of onions, carrots and celery (either common Pascal celery or celeriac). Mirepoix, either raw, roasted or sautéed with butter, is the flavor base for a wide number of dishes, such as stocks, soups, stews and sauces.
These three ingredients are commonly referred to as aromatics. Similar such combinations, both in and out of the French culinary repertoire, may include leeks, parsnips, garlic, tomatoes, shallots, mushrooms, bell peppers, chiles, and ginger. For the combination mirepoix au gras, or a Matignon, ham and/or pork belly are used as additional ingredients.

They may be used in various combinations, as dictated by the cuisine and the dish itself.

Traditionally, the ratio for mirepoix is 2:1:1 of onions, celery, and carrots. The ratio for bones to mirepoix for stock is 10:1. When making a white stock, or fond blanc, parsnips are used instead of carrots to maintain the pale color.