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 until the casserole is heated through and the cheese is melted, for about 15 minutes.
Allow to rest for 5 minutes before serving.
Video
Notes
It's fine to use rotisserie chicken in this recipe. Both the white and dark meat will work, and you can either shred or dice the meat. But do make sure to completely remove the skin. Its texture won't work in a casserole.
Sometimes, when I pull it out of the oven, I like to 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 the fridge, in an airtight container, for 3-4 days. Reheat them in the microwave, covered, at 50% power. You can also freeze individual portions for up to three months.