Make a roux by heating oil in a heavy bottomed cast iron pot over high heat. Whisk flour in to the hot oil. Reduce heat to medium and continue whisking until the roux turns deep brown, about 15 minutes.
Add onions to the roux and stir. Reduce heat to medium low and continue until the roux is a dark brown, about 10 minutes.
Add the smoked sausage and stir for 1 minute.
Add the celery, bell peppers, onion, garlic, Cajun seasoning, garlic powder and onion powder. Cook, stirring for 3 minutes. Add the thyme, seafood stock and bay leaves. Bring the gumbo to a boil, stirring occasionally. Reduce heat to medium low and simmer for 45 minutes. Stir occasionally and skim off the fat.
Add Worcestershire, file powder and Tabasco. Season with salt and pepper to taste. Simmer for another 45 minutes, continuing to skim the fat off the top Add seafood and cook for 10 minutes.
Remove the bay leaves and serve over rice, garnish with parsley and green onions.