In a large, heavy pot over medium heat, cook the onions, bell peppers, celery, and garlic in oil until wilted, about 5 minutes. Add the dry roux and fish stock (or other liquid), bring to a boil, and stir to combine well. Add the tomatoes and season to taste with salt, black pepper, and cayenne. Reduce the heat and simmer for about one hour. Add the fish and cook another 15 minutes. Adjust seasonings, add green onions, parsley, and sliced lemons. Serve over rice.
Makes 4 servings