Mirepoix is similar to bouquets, sachets and oignon brules in the fact that they are used to added subtle flavors to enhance a dish. One chef I used to work with claimed this was the necessary start to every soup in order for it to taste balanced. A miropoix is usually a blend of aromatic vegetables, most commonly a mixture of 2 parts onion, and 1 part each of celery and carrot. This mixture is sometimes changed to include leeks, parsnips, tomatos, garlic, mushrooms, shallots, peppers, and ginger.

A pound of mirepoix is usually enough to add flavor to one gallon of stock.

Different styles and regional cooking can require a different style of mirepoix. When making a white sauce you may wish to use a white vegetable mirepoix of 2 parts onion, 1 part celery root and one part parsnips. In Asian dishes you may want to have a mirepoix of 2 parts ginger, 2 parts garlic, and 1 part scallion. The cajun trinity is 2 parts onion, 1 part pepper, and one part celery.

After cleaning the vegetables, you can decide whether or not you wish to peel them. Onion skins can give the liquid a yellowish orange tint. Scrubbing, and not peeling the vegetables can reduce prep time. Cut the vegetables in uniform sizes based on the cooking time of the dish, if the dish is cooking for a short time a smaller mirepoix will be necessary to extract the necessary flavors. Large cut vegetalbe are best for dishes like pot roast and demi-glace.

CIA suggests starting with suating the onions, then the carrots in fat like butter (Italian versions use olive oil and are called soffrito). Once the onions and carrots are starting to sweat or turning brown add the celery. If you place a lid on the pot while sweating the mirepoix it is a technique called smothering. Some recipes suggests adding tomato paste to the mirepoix after it has been sweating to add flavor and color. If adding tomato paste cook until the paste starts to turn brown. This technique is called pincage, which means to stiffen or pinch.

Chef Daniel Boulud suggests using a mirepoix when braising meats. The mirepoix is seared with the meats, then pan roast it until its lightly carmelized. After the dish has been pan roast to carmelize the dish is drain of fats, flour added, and then the dish is cooked in red wine or other braising liquid.