Mirepoix is the French name for a combination of onions, carrots and celery (either common Pascal celery or celeriac). Mirepoix, either raw, roasted or sautéed with butter, is the flavour base for a wide number of dishes, such as stocks, soups, stews and sauces. Mirepoix is known as the holy trinity of French cooking.
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, chillies, and ginger. For the combination mirepoix au gras, or a Matignon, ham and/or pork belly are used as additional ingredients.
To make a white mirepoix for a white stock, or fond blanc, parsnips are used instead of carrots to maintain the pale colour. Celeriac can be substituted for the celery for an even whiter mirepoix and it can be a good idea to include the whites of leeks in place of part of the onions in a white mirepoix as they give an excellent flavour. However, in professional kitchens many chefs often prefer to use a standard mirepoix rather than white mirepoix for all stocks.
Cutting the vegetables
Chop the vegetables coarsely into pieces of relatively uniform size. As mirepoix is rarely served, it is not usually necessary to cut it neatly. The size depends on how long the mirepoix will cook. If it's going to take a long time, as for beef stock, cut the vegetables into large pieces (3 to 5 cm). Cutting into small pieces is necessary for releasing flavours in a short time, such as when the mirepoix will be used for fish stock.
In this example, the ingredients are brunoised.
In addition to the onions in the mirepoix, an oignon brûlé (“burnt onion”) is sometimes added to brown stock to give it colour as well as flavor. To prepare, cut a large onion in half crosswise and place it, cut side down, on in a heavy frying pan. Cook until the cut surface is dark brown. Add to the stock.
Oignon piqué (or clouté)
Another form of onion for flavouring is the oignon piqué ("pricked onion") or oignon clouté ("studded onion"). This is used not so much for stocks but for soups and sauces. To prepare, pin one or more bay leaves to a whole, peeled onion with the aid of a whole clove. Adding the bay leaf and clove attached to the onion makes removing them easier when cooking is finished.