Wash the tomatoes and cut them in half across their width. Scoop out the seeds with a teaspoon to partly hollow out the tomatoes. Do not squeeze them. Place them, cut side up, on a baking pan large enough for them to fit in one layer, snugly if necessary but without overlapping.
Combine in a small bowl the ham, chopped onion, olives, parsley, grated Parmesan, 4tablespoons of olive oil, salt, and very liberal grindings of pepper and mix the ingredients thoroughly with a fork.
Stuff the hollowed tomatoes with the ham and olive mixture, distributing it evenly among them. It is all right for it to form a mound. Sprinkle the bread crumbs over the stuffed tomatoes and trickle the remaining tablespoon of olive oil over them. Bake in the upper level of the preheated oven for about 30 to 35 minutes until the tops of the tomatoes become colored a very light brown. Serve while still warm, but not piping hot.