Preheat the oven to 200 degrees C. Push a metal skewer through each potato. (The metal skewer helps to conduct heat through to the centre of the potatoes so that they cook more quickly.) Put the potatoes directly on the shelf in the oven and bake for about 1 1/4 hours or until they are tender.
Make the filling when you first put the potatoes in the oven so that it has time to marinate. Alternatively, it can be made just before the potatoes are cooked, and served hot. Heat a large ridged chargrill pan or frying pan. Drizzle half the oil over the pan and cook the mushrooms and zucchini slices for 10–15 minutes or until they are tender and well browned in places and have released their juices.
Transfer the vegetables to a bowl with all their juices and add the remaining oil, the vinegar and mustard. Add pepper to taste and mix well, then leave to marinate until the potatoes are cooked.
Split open the baked potatoes, then press gently to part the halves, keeping them joined at the base. Stir the parsley into the marinated vegetables, then pile them into the potatoes. Serve immediately.