Preheat the oven to 350°F (175°C). Lightly spray two mini cast-iron skillets with nonstick spray or line a baking sheet if using one.
Wipe the mushrooms clean with a damp cloth. Remove and discard (or save) the stems by gently twisting and pulling so each cap is hollowed.
In a mixing bowl, combine the softened cream cheese, grated Parmesan (or mozzarella), dried basil, dried oregano, garlic salt, and black pepper until smooth.
Fold the chopped pepperoni, green pepper, red onion, and black olives into the cheese mixture until evenly distributed.
Spoon the filling into the mushroom caps by heaping teaspoons, filling each cap generously.
Arrange five stuffed mushrooms in each prepared mini skillet (or place them on the baking sheet) and bake for 20 minutes.
After 20 minutes, switch the oven to broil and broil briefly until the tops are golden and bubbly, watching closely to prevent burning. Remove and serve hot.