Unlike French Mirepoix, which uses a 1:2:1 ratio of carrots to onions to celery, the Cajun Holy Trinity is made from equal parts celery, onions, and bell peppers. The coarseness of the vegetables is dependent on the mother recipe.
Cajun cuisine is famously inspired by French methods and flavors, as French Catholics and Acadians (or “Cajuns,” as pronounced by English-speaking Louisianans’) settled in New Orleans in the early 18th century. The settlers quickly realized that, while the Louisiana ground wasn’t suitable for growing carrots, it was ideal for growing bell peppers.