1. Roll the fillets and secure with toothpicks. In a skillet heat the butter, add the onion and cook until it is transparent.
2. Arrange the fillets in the pan and pour the fish stock and wine over them. Add the tomatoes and parsley and bring to a boil. Reduce the heat and cook gently until the fish flakes easily when tested with a fork, ten to twelve minutes. Turn carefully once while cooking.
3. Remove the fillets to a serving dish and keep warm. Stir the sauce over high heat two or three minutes, until slightly reduced. Add the pepper and pour the sauce over the fillets.