Mix the prosciutto with the parsley, bread crumbs and cheese. Add enough olive oil to bind the mixture. Spoon a little of the filling into each of the mushroom caps.
Spoon three-quarters of the marinara sauce over the bottom of a baking dish and arrange the mushrooms over it. Spoon the remaining sauce over the stuffed mushrooms. Bake for thirty minutes. Serve hot or cold as antipasto.