Preheat the oven to 400°F. Lightly grease a 2-quart casserole dish.
In a large bowl, mix the shredded chicken, diced pepperoni, pizza sauce, garlic powder, dried oregano, red pepper flakes, 1 cup of mozzarella, and parmesan.
Transfer the mixture to the prepared baking dish.
Top the casserole with the remaining ¾ cup of mozzarella cheese and the pepperoni slices.
Bake the casserole until it's heated through and the cheese is melted, for about 15 minutes. Allow it to rest for 5 minutes before serving.
Video
Notes
It's fine to use rotisserie chicken. Both the white and dark meat will work, and you can either shred or dice the meat. But please remove the skin. Its texture won't work in a casserole.
Sometimes, when I pull it out of the oven, I sprinkle the casserole with dried parsley or oregano and some more red pepper flakes. This is purely for garnish, but it does look pretty.
You can keep the leftovers in an airtight container in the fridge for 3-4 days. Reheat them covered in the microwave at 50% power. You can also freeze this casserole. I like to divide it into individual portions and freeze them in freezer-safe containers. They can stay in the freezer for up to three months.