Mirepoix

Its all in the ratio

Most cookbooks explain mirepoix like this: 1 part carrot, 1 part celery, 2 parts onion.  It is way easier to understand as 1 small onion, 1 celery stalk, 1 carrot.  Nature portions things out nicely without measuring cups.  As for mirepoix's usefulness?  Look it up, its pretty useful.