Preheat oven to 375 degrees. Lightly spray a cast iron skillet or a baking sheet. Set aside.
Wash mushrooms carefully with a damp cloth. Remove stems from mushrooms and save for a later use.
In a small bowl, add cream cheese, grated cheese and Italian seasoning. Stir until well combined. Add in chopped pepperoni and chopped onion. Stir well to mix.
Using a small spoon, fill each mushroom cap with the cream cheese mixture. Place in prepared pan and bake for 15 minutes. Switch oven to broil and broil for a couple minutes just until the tops start to brown.