Lightly saute the onions and garlic in the oil until they are soft and golden. After a few minutes add the sliced mushrooms. Cut off the lid of the tomatoes and scoop out the interior, discarding the seeds but reserving some of the flesh. Chop the flesh into small dice and add it to the onion/mushroom mix. The mix should be moist but not soggy. Add the cheese, parsley and ham and season and stuff back into the tomato shells. Sprinkle with olive oil and bake for 20-30 minutes in a hot (180C/350/gas 5) oven in a tray that will support the tomatoes without collapsing.