Preheat the oven to 375. Spray a high-sided 9×13 baking dish with olive oil cooking spray, then spread 1 cup of marinara evenly on the bottom of the pan. Cut each bell pepper in half from top to bottom. Remove the seeds and membranes, then arrange cut side up in the baking dish.

Heat a large high-sided skillet over medium heat. Add the olive oil, diced onions and ¼ teaspoon salt. Cook 3-5 minutes, or until the onion begins to soften, stirring occasionally.

Add the ground beef and sprinkle with ½ teaspoon salt, Italian seasoning, garlic powder, and red pepper flakes. Break into small pieces and cook until browned and cooked through, about 5-7 minutes.

Turn off the heat, then add the remaining 1 cup marinara, rice, parsley, parmesan, and ½ cup Italian cheese, stirring until just combined.

Fill each pepper evenly with the beef rice mixture, about one heaping ½ cup for each pepper. Sprinkle over the remaining ½ cup cheese. Cover tightly with foil and bake for 30-35 minutes, then uncovered for 5-10 minutes until the cheese is browned and bubbling.