Preheat the oven to 200C, 400F, Gas 6 Brush a little olive oil over each mushroom and arrange, upside down, on a non stick baking tray.
Mix the tomatoes, onions,garlic,cheese, olives and parsley together and season with salt and freshly ground black pepper.
Divide de mixture equally between each mushroom. Transfer to the preheat oven and bake for 15 minutes or until the mushrooms are tender and the filling is pipping hot.