Preheat the oven to 190C/375F/Gas 5. Slice the tops off the tomatoes, then scoop out the centres, remove the seeds and juice and finely chop half of the flesh, discarding the rest.
Mix together the breadcrumbs, garlic, parsley, Parmesan and seasoning. Heat half of the oil in a frying pan and quickly sauté the mushrooms. Stir in the tomato flesh and most of the breadcrumbs.
Place the tomatoes in an oiled baking dish and stuff with the mushroom mixture. Sprinkle with the rest of the breadcrumbs and drizzle over the remaining oil. Bake for 20-25 minutes until lightly browned and sizzling. Serve hot.