One of the first tasks you learn in cooking class is knife skills. To get sufficient practice, you slice, julienne, chop, dice, and mince a lot, and one of the first things beginning students hone their skills on is mirepoix (pronounced meer-pwah.) This mixture of onion, carrot, and celery is one of the major flavor bases of French cooking.
Depending on who you ask, the size of the cubes varies. My teacher called for ¼ to 1/6″ cubes. Some chefs insist it be 1/8″ cubes. For sauces, as long as the cubes are small enough to provide a lot of surface area to extract maximum flavor, anything in this range is suitable. For long-cooking braises, a large chop will suffice. If the mirepoix is to remain in the finished dish, and appearance is important, be sure your chop is consistent. As I said, great practice for your knife skills!
Whichever size cubes you produce, proportions are important. Think about the properties of these vegetables (which in a mirepoix, are actually being used as aromatics, or flavorings.) Carrots contain a lot of sugar, so too much carrot will make a mirepoix that’s sweet. Celery can get overly vegetal or bitter, so you want to go easy on that. Onions mellow with gentle cooking, so you can use more of those. A happy balance is two parts onion, and one part each of carrot and celery.
What does one do with a mirepoix? It’s a base for sauces like béchamel, which can be bland and one-dimensional without it, and espagnole, the basic brown sauce of French cooking. It’s used to create a flavor base for soups, stews, and braises, or as a bed on which to bake or poach fish.
Depending on where in France, the fat used to cook the vegetables may be butter (the classic choice), olive oil (in Provence), goose or duck fat (in Gascony), or even lard (in Alsace, where there’s a heavy German influence.) Sometimes a bit of ham may be added for additional flavor; bay leaf and thyme sometimes find their way into it. There’s a lot of leeway.
Other cuisines have their flavor bases, too. Italy has its soffritto, which may contain garlic, pancetta or prosciutto, sometimes fennel or sage. Spain and Puerto Rico spell it almost the same way, sofrito, but use their own flavors. It all depends on what the traditional meat or fat and vegetables are, but onion is almost universal. Tomatoes find their way into some flavor bases, as do ginger and galangal, chiles, orange zest, almonds, seeds, and many different spices.
Here in the U.S., the flavor base that stands out to me is the “holy trinity” of Creole and Cajun cooking, onion, celery, and green pepper. Add garlic to it, and it’s the flavor base I use for seasoning chili. I recently learned that a number of chefs put carrots in their chili – heresy, to my mind! But then, chili is open to a lot of interpretation.
Flavor bases should be cooked gently. If you burn the vegetables – and cut into small dice, too high a heat will quickly scorch them – it will ruin the flavor of a dish. The proper term is to etuver, or “sweat” a mirepoix. Low heat and a bit of salt to draw the juices ensure a flavorful result. How long the dish will cook will determine in part how long to cook a flavor base. If the total cooking time is not long, you want to be sure the base is tender before the rest of the ingredients go in. If it’s a long-cooking dish like a braise, less time is needed.
How can you apply this to your cooking? Think about a vegetable soup. If all you were to do is cut up and boil the desired vegetables, the resulting soup will be pretty one-dimensional. If, on the other hand, you begin by making a mirepoix to create a flavor base (remember, fat draws out flavor), then add the vegetables and either or stock (stock will add another layer of flavor), you’ve elevated that soup into another realm.
Each element you pick will contribute its own unique flavor to the end product. I make a cream of carrot of soup with both leeks and onions in the base. Students who have difficulty finding leeks have made it with onions only. It’s still good, but it’s different. They ask what will happen if they were to add curry to the base. I tell them, try it and see. (It’s good, but it’s no longer French.) What if you added tomatoes to the base? I haven’t tried it, but I’d guess not so good. On the other hand, carrots go into the base of an Italian ragù I make which has tomatoes in it, and it’s delicious. The permutations are endless – have fun experimenting.