Preheat oven to 400℉
Prepare peppers: wash peppers, then cut them in half lengthwise from the top down. Remove the stem if you wish. Remove seeds and membranes.
Optional: If you like your stuffed peppers more tender, pre-cook the peppers. Microwave peppers for 3 minutes or parboil for 3-5 minutes.
Make the stuffed pepper filling: in a large skillet heat 1 tablespoon olive oil over medium heat. Add onion and sauté until the onions are tender and translucent.
To the pan, add in cooked chicken, cooked rice, 2 cups marinara sauce, and season with salt, black pepper, chili powder, onion powder and Italian seasoning.
Stir in ½ cup of shredded cheese and reserve the remaining cheese for topping the peppers. Simmer the stuffed pepper mixture for 5 minutes.
Pour 1 cup of marinara sauce on the bottom of a 9x13 inch casserole dish.
Arrange the pepper halves in a single layer in the bottom of the casserole dish. Season the insides of the peppers with salt and black pepper.
Stuff the peppers - divide the meat mixture evenly among the 8 pepper halves.
Top with remaining cheese, then cover the casserole dish with foil.
Bake: bake covered for 15 minutes, remove the foil and bake for another 10 minutes or until the cheese is melted.
Garnish with chopped fresh parsley.