Preheat the oven to 190°C/ 375°F/Gas mark 5.
Put the tomatoes in a bowl of justboiled water for 30 seconds. Peel off the skins and chop the flesh.
Remove the mushroom stalks. Chop the stalks; mix with the tomatoes, grated onion, butter and parsley. Season with salt and pepper.
Put the mushrooms cap side down in a large greased ovenproof dish. Fill each one with the stuffing.
Cover with foil, and bake in the oven for about 30 minutes.