Preheat oven to 375 degrees f.
In large oven-proof cast iron, or flat rimmed sheet pan, lay out mushrooms upside down as shown.
Fill each mushroom cap with about a Tablespoon of marinara, (I sprinkled mine very lightly, and equally with garlic powder, Italian seasoning, and sea salt at this point) and sprinkle with mozzarella.
Top each with pepperoni if desired, and bake until cheese is melty, for about 15 to 20 minutes.
Sprinkle with fresh chopped parsley or basil.