Combine the flour and oil in a large cast-iron or enameled cast-iron Dutch oven over medium heat. Stirring constantly for 20 to 25 minutes, make a dark brown roux, the color of chocolate. Season the onions, bell peppers, and celery with the salt and cayenne. Add them to the roux and cook, stirring occasionally, until soft, about 5 minutes. Add the broth, bay leaves, and chicken. Bring to a boil. Reduce the heat to medium-low and cook, uncovered, stirring occasionally, for 2 hours. Skim off any fat that rises to the surface and continue to simmer for 30 minutes.
Add the oysters and the oyster liquor. Cook for 2 to 3 minutes, or until the edges of the oysters curl.
Remove from the heat. Remove the bay leaves. Add the file powder and parsley.
Serve in soup bowls with rice.
Yield: 6 servings