Preheat your oven to 325°F.
Clean mushrooms with a damp cloth and carefully remove the stems.
Set the stems aside.
In a large skillet, melt butter over medium heat.
Sauté onion, garlic, pepperoni, green pepper, and mushroom stems until they’re tender.
Keep them from browning.
Remove the skillet from heat.
Incorporate bread crumbs, Parmesan, salt, and black pepper into the mixture.
Mix everything well.
Fill each mushroom cap with the stuffing mixture.
Drizzle with olive oil.
Sprinkle mozzarella on top.
Bake the stuffed mushrooms for 20 minutes or until the cheese is golden.
Serve hot.