Preheat oven to 350 degrees F. Line a baking sheet with foil or use a mini muffin tin to cook the mushrooms.

In a large skillet, cook the sausage, pepperoni, and the onion over medium heat for 5-6 minutes or until the sausage is no longer pink. Drain.

Reduce heat to low. Add the cream cheese, pizza sauce, and oregano to the skillet. Stir and cook until melted. Stir in the Parmesan and remove from the heat.

Spoon a heaping tablespoon of mixture into each mushroom cap (a cookie dough scoop works well here) and place on baking sheet.

Bake mushrooms for about 15 minutes or until mushrooms are tender. Remove from oven and top the mushrooms with mozzarella and additional chopped pepperoni (for looks) if desired. Return to the oven for 2-4 minutes or until cheese is melted.