Mirepoix (a French cooking term [meer-pwah]) is a mixture of finely chopped/diced vegetables used to enhance flavor of a dish. It can be raw, roasted or sautéed with butter or oil. The purpose of mirepoix is to give a subtle background flavor or it serve as a base to a wide variety of dishes, such as stocks, soups, stews and sauces.

Traditional ratio is two parts onions, one part carrots, and one part celery and a bay leaves. I find it that celery can be bit overpowering and I prefer sweeter flavor of onion and carrot. So I’d like to use Mirepoix as follows:

3 parts – onion
2 parts – carrot
1 part  – parsnip or green celery or celeriac
bay leave is optional – depends on dish I make

and 3-2-1 is easier to remember :)
Aromatic vegetable are finely chopped or diced into uniform pieces and sautéed on low heat. The shorter the cooking time of your recipe, the smaller the pieces should be, so that they effectively infuse the foods with flavour.