Set a large heavy pot or Dutch oven on the stove over medium heat.

Add the oil and flour to the pot and whisk until smooth.

Cook the flour-and-oil mixture, stirring almost constantly, for 10 to 12 minutes, until it is the color of peanut butter. If dark bits start to appear on the bottom, lower the heat a little.

Add the chopped onion, bell pepper, and celery to the pot.

Cook the vegetables for 5 to 6 minutes, stirring often, until they soften and the dark flour mixture coats them evenly.

Add the garlic and Cajun seasoning to the pot.

Cook for 1 minute, stirring, until the garlic smells fragrant.

Pour 1 cup of the vegetable broth into the pot.

Stir well and scrape the bottom of the pot until the dark mixture loosens into a smooth base.

Pour the remaining 5 cups vegetable broth into the pot.

Add the lentils to the pot and stir.

Bring the pot to a boil over high heat.