Make the sauce: In a large saucepan, heat the oil over moderate heat. Add the scallions, garlic, celery, green pepper, and onions. Cook, stirring frequently, about 5 minute until softened but not brown. Add the flour to the veg. mixture and stir to combine. Continue to cook, stirring, about 1 minute longer. Add the tomatoes, lemon juice, worcestershire sauce, thyme, cayenne, black pepper, and stock. Bring to a boil, stirring, then simmer, partially covered, 25-30 minute Stir the parsley into the sauce, then remove about 1 cup of the sauce from the pan and reserve. Add the fish fillets to the pan in a single layer. Pour the reserved sauce over the fillets. Partially cover the pan and simmer the fish about 10 minutes, until the fish flakes. With a large slotted spatula, transfer the fish to a serving dish and spoon the sauce over the fillets. Serve with creamy mashed potatoes or rice and green beans.