In a large pot (8-quart capacity) heat 1/4 cup of the oil. Saute onions, celery, and carrots for about 5 minutes until onions become translucent and celery and carrots begin to soften. Add broth and bring to a simmer. Add fish, oysters, Cajun seasoning, tomatoes, bay leaves, and salt and pepper, and cook until fish becomes opaque. In a separate bowl, mix the other 1/2 cup of oil with the flour, and stir this mixture gently into the stew to thicken. Cook for a few minutes more to integrate flavors.