Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl, mix together mozzarella, cheddar, and Parmesan cheeses until well combined.
Add Italian seasoning and garlic powder to the cheese mixture, stirring to combine; add red pepper flakes if desired.
If pepperoni slices are large, cut them in half for easier handling.
Arrange pepperoni slices in a single layer on the prepared baking sheet, spacing them slightly apart.
Sprinkle a generous amount of the cheese mixture evenly over each pepperoni slice.
Bake for 8–10 minutes until cheese is melted, bubbly, lightly golden, and pepperoni is crispy.
Remove from oven and let crisps cool on baking sheet a few minutes, then transfer to a wire rack to cool completely.
Serve immediately, garnished with fresh chopped parsley if desired.
Store leftovers in an airtight container at room temperature and reheat in a toaster oven or air fryer to crisp.