While the pastry is cooking, prepare the filling. Heat the olive oil in a large frying pan, add the onion, cook for 5-6 minutes over a medium heat until softened. Chop the porcini mushrooms and add to the onion with the garlic, stir together for 1 minute. Slice any large mushrooms then add them all to the pan and fry a for further 2 minutes. Spoon into the pastry cases.