In classical French cooking, the aromatic base is a mirepoix: two parts onion, one part carrot, one part celery. In Cajun and Creole cooking, the carrot is replaced with green bell pepper, giving a lighter, sweeter flavor profile that suits the spices and proteins of Louisiana cuisine.
The standard ratio for the Cajun Holy Trinity is two parts onion to one part celery to one part bell pepper. Some cooks add a fourth element -- garlic -- which has led Cajun cooks to call it "the Pope" since it makes the Trinity complete.
How to Cut It
For most Cajun dishes, dice the Trinity into a small, consistent dice -- roughly 1/4 inch. Inconsistent cutting leads to uneven cooking: some pieces will be raw while others are overcooked. Take the time to cut everything the same size.
How to Cook It
In a roux-based dish, the Trinity goes in immediately after the roux reaches the right color. In dishes without roux, cook the Trinity in butter or oil over medium heat for 8-10 minutes until the onion is translucent and the edges begin to brown. Do not rush this step. Properly cooked Trinity is sweet, soft, and deeply aromatic -- the foundation of everything that follows.