Preheat the oven to 400º. In a large bowl, combine 2 cups of mozzarella, Parmesan, garlic, marinara, parsley, and red pepper flakes, and season with salt and pepper. Stir until combined, then gently fold in chicken.
Spoon mixture into halved bell peppers and sprinkle with the remaining 1 cup of mozzarella.
Pour chicken broth into the baking dish (to help the peppers steam) and cover with foil.
Bake until peppers are tender, 55 minutes to 1 hour. Uncover and broil for 2 minutes.
Garnish with parsley and more Parmesan before serving.
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