Just as mirepoix (finely chopped onion, celery, and carrot) is a foundational ingredient of countless French dishes, so is the combination of onions, celery, and green peppers in the Cajun and Creole cuisines of Louisiana. Known as the holy trinity, the three elements are finely chopped and usually sautéed in butter to become the base for jambalaya or étouffée or, when blended with a bit of flour, the roux that starts a gumbo.