Heat the oil in a large saucepan, add the red and green peppers, mushrooms, zucchini, eggplant, tomatoes and tomato purée and fry for 15-20 minutes over a moderate heat, stirring occasionally.
Meanwhile, line a baking sheet with a large piece of foil, allowing sufficient to fold over comfortably. Place the fish on the foil, dot with butter, season with salt and pepper and fold the foil loosely over the fish. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 18—20 minutes until cooked through.
When the fish and vegetables are cooked, season to taste and add more herbs or tomato purée if necessary. Spoon the vegetables on to warmed plates and put the halibut on top. Serve immediately.