Whisk the oil and flour together in a heavy pot over medium heat to start a roux.

Cook the roux, whisking constantly, for 20 to 30 minutes, until it turns the color of melted dark chocolate.

Add the onion, bell pepper and celery straight into the hot roux and stir for 3 minutes, until slightly softened.

Stir in the garlic and cook 30 seconds more, until fragrant.

Whisk in the chicken broth 1 cup at a time, making sure each addition is smooth before adding the next.

Add the chicken, andouille, bay leaves, Cajun seasoning, thyme and cayenne, and bring to a gentle boil.

Reduce the heat to low and simmer, uncovered, for 45 minutes, until the chicken is fork-tender and the broth coats the back of a spoon.

Stir in the okra, if using, during the last 10 minutes, until tender but not mushy.

Remove the bay leaves, taste, and season with salt and black pepper.

Ladle the gumbo over rice in each bowl and finish with file powder and sliced green onion.