Cajun and Creole cooking often begin with chopped onion, celery and green bell pepper. PawPaws and MawMaws of French origin in Louisiana have often called this mix the "trinity." If you add chopped carrots, garlic, and scallions to that mix you then have what's referred to as " mirepoix" (pronounced meer-a-pwah).
Everybody has different measurements for these items, but for the most part, it consists of equal parts green bell pepper and celery, then twice that amount of onion. A mirepoix, has equal amounts of bell pepper, celery and carrots and twice as much onion, plus half again as much garlic and scallions. But that's the basics...it's perfectly acceptable to configure your own proportions.
Oh, yeah....always use a sharp knife--not a food processor--to prep your trinity or mirepoix. But be careful not to cut your fingers! Chop the ingredients but don't leave them in big chunks. Nice and small and uniform is the best. Get some friends involved in the chopping and freeze what you make in freezer 'cause you gonna use it all the time in Louisiana cooking.