Arrange the fish fillets in a large ovenproof dish, brush with sunflower oil and place under a preheated moderate grill for 5 minutes on each side. Cover the fish and keep warm.
To make the mushroom sauce, heat the oil in a saucepan. Add the onion and fry for 3 minutes, stir in the mushrooms and cook for 5 minutes until softened. Stir in the walnuts, parsley and wine, season with salt and pepper and bring to the boil. Lower the heat and simmer for 5 minutes. Add the tomatoes and simmer for 2 minutes.
Spoon the sauce over the fish and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes, until the fish is just firm. Serve at once on a bed of crisp green vegetables.