Arguments against innovation always lose. One might, if a good orator, make a case for romance, but challenging expediency or questioning the value of change—these points are futile. No one likes a cornered luddite, anyway. There is, however, a single exception: two parts onion, one part carrot, one part celery. This is the noble ratio of mirepoix, and it shoos away trifling attempts to innovate like fruit flies drunk on house wine.
The ratio, 2:1:1, is crucial to maintaining balance. Each component contributes something specific: onions, the familiar and rounded savory character; carrots, sweetness and body; celery, a dry vegetal note. But the ratio alone won't guard against imbalance; one must also choose wisely within each component. The onions should be medium white ones. Yellow onions are too sweet, purple ones turn things pink. The carrots should be mature; baby carrots are best appreciated raw. The celery should be mature as well, but the outermost stalks must be peeled of strings. Strict? Yes. But that’s how it goes when in pursuit of consistent harmony.
Size must be carefully controlled. Intended cooking time determines how large or small the mirepoix should be chopped. Quick broths, for poaching, say, a few pieces of salmon, need as fine a dice as possible so as to maximize extraction. Sauces, if they are to cook for more than forty minutes, need a medium dice. Soups and braises require a dice large enough to resist disintegration during the long stretches of simmering. Stocks, which can cook for several hours, call for no more preparation than splitting the onions, carrots and celery stalks lengthwise. Those large pieces, by the way, are the prize for the cook who has spent all day making stock; eaten with rock salt and butter, they are delicious.
Making mirepoix (and peeling potatoes) is how the youngest apprentices earn their daily meals in the traditional French brigade system. It's not a task soon forgotten. In part, this is because making mounds of mirepoix is labor intensive. To the kitchen hand, it is an involved, back breaking, finger-nicking method for creating flavor. By the time those novices begin using the fruits of their labor, a reverential respect has grown. The home cook might not have this advantage, so it is understandable that mirepoix might be viewed with skepticism. This is perhaps where innovation creeps in: why chop all those boring vegetables when this sexy new seasoning exists? The answer, of course, is because mirepoix is the very matter on which most of western cookery is based. It is the universal taste of savory. It is the incorruptible holder of all culinary debt.