Preheat oven to 350 degrees F. In a medium-sized skillet saute oil, butter, and onion over medium heat. Cook, stirring occasionally, until onions are translucent, about 5 minutes.
Add garlic, pepperoncini, and prosciutto. Cook 2 minutes, stirring constantly for 2 minutes. Add tomato and cook 2 to 3 minutes, stirring occasionally. Add bread crumbs, remove from heat, and stir until crumbs are thoroughly mixed in and moistened.
Add ½ cup Parmesan cheese; stir until incorporated.
Firmly pack a generous tablespoon of mixture into each mushroom. Sprinkle tops with remaining Parmesan cheese and bake 20 minutes.