Make the Roux
Heat oil in heavy 6-quart pot over medium-high heat until very hot and almost smoking. Add flour and stir constantly until mixture is a deep, clay red - well past "peanut butter", but not burnt/dark red/dark brown either. Reduce the heat as you go -- about 15 minutes.
Add the Trinity
Add 3/4s of the chopped onions, chopped bell peppers, and chopped celery. Cook until onions are soft and brown, stirring frequently, about 15 minutes. Add garlic, cayenne, and cajun seasoning, and stir 2 minutes.
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