The term mirepoix refers to a combination of diced vegetables - typically carrots, onions and celery. It is the classic French flavor base of many stocks, sauces, stews and soups. It is often used as a bed for braising meat. The approximate proportions for a mirepoix are 50 percent onions, 25 percent carrots and 25 percent celery.
To make a white mirepoix, typically for a fish stock or fumet (concentrated stock), use leeks. Garlic may also be added to a cooking, or caramelizing, mirepoix, but always toward the very end of cooking, as garlic burns easily. The size of mirepoix cut depends on the length of time and method by which the vegetables will be cooked; use fine dice for a quick-cooking dish and coarse, larger cuts for longer-cooked dishes, such as beef stock or pot roast. Add the vegetables directly to liquids, or caramelize in butter first to bring out their natural sweetness.