Make the Roux:

In a large pot or Dutch oven, heat the oil over medium heat. Gradually whisk in the flour, stirring continuously, to create a roux. Cook the roux, stirring constantly, until it reaches a dark brown color, about 20-30 minutes.

Sauté the Vegetables:

Add the onion, bell pepper, and celery to the roux. Cook until the vegetables are softened, about 5 minutes. Stir in the garlic and cook for another minute.

Cook the Sausage and Add Stock:

Add the sliced Andouille sausage to the pot and cook for about 5 minutes. Pour in the chicken or seafood stock, scraping up any bits from the bottom of the pot. Add the bay leaves, thyme, oregano, and Cajun seasoning. Bring to a simmer and cook for 30 minutes.

Add the Shrimp:

Stir in the shrimp and cook for an additional 5-7 minutes, or until the shrimp are pink and cooked through.

Final Seasoning and Herbs:

Remove the bay leaves. Season the gumbo with salt and pepper to taste. Stir in the chopped parsley and green onions.

Serve:

Serve the gumbo hot over cooked white rice. Sprinkle with file powder if desired.