Preheat your oven to 200 C (390 F). Get out a 9x13 baking dish.
Heat a skillet with a splash of olive oil and add the cubed chicken, season with salt and pepper as desired.
Cook until just cooked through for tender chicken, drain any oil, I use a colander for this and just let the chicken sit in the colander until it is needed.
Cut bell peppers in half and remove the stem and seeds. Place in a 9x13 baking dish, fit them in there tight.
In a large bowl, combine 2 cups Mozzarella, Parmesan, garlic, pasta sauce, parsley, and red pepper flakes, and season with salt and pepper to your liking. Stir until combined, then gently fold in chicken.
Spoon chicken mixture into halved bell peppers and sprinkle with remaining 1 cup Mozzarella cheese.
Pour chicken broth into baking dish to help the peppers steam and cover the dish with foil.
Bake until peppers are tender, about 1 hour. Uncover and broil 2 minutes.
Garnish with fresh chopped parsley and Parmesan cheese, serve.