Preheat oven to 400 degrees F. Place oven rack in middle of the oven. Lightly grease a shallow baking dish or baking sheet with sides (large enough to hold the mushroom caps in one layer).
To remove stems from mushrooms, hold each mushroom stem side up in the palm of one hand and use the other hand to push stem gently sideways until it pops out. If any stem remains in the cap, gently scoop it out with a small spoon. Trim off and discard bottoms of stems and then finely chop the stems.
In a large frying pan over medium heat, heat 2 tablespoon olive oil. Add chopped mushroom stems, garlic, and onion. Cook stirring, until the stems are very tender, approximately 8 minutes; remove from heat and transfer to a large bowl.
Add the prosciutto, bread crumbs, parsley, 1/2 cup parmesan cheese, egg, salt, and pepper; stir to combine well.
Spoon the filling among the mushroom caps, mounding it slightly. Arrange the stuffed mushroom in one layer on the prepared baking dish. Sprinkle mushrooms with remaining 1/4 cup Parmesan cheese and drizzle with the remaining 2 tablespoon olive oil.
Bake until the mushroom caps are tender, approximately 20 minutes. Remove from oven and serve immediately.
Makes 24 stuffed mushrooms.
Prosciutto and Parmesan Stuffed Mushrooms Recipe: https://whatscookingamerica.net/lindasandberg/appetizers/prosciuttostuffedmushrooms.htm