Make the marinara sauce. In a 10 or 12-inch skillet, heat 2 tablespoons of olive oil over medium-low heat. Then, add the chopped onion and cook for 5 minutes, stirring occasionally to prevent burning. Next, add minced garlic and cook for 1 more minute. Stir in crushed tomatoes, dried basil, oregano, garlic powder, salt, and pepper. Lower the heat and let the sauce simmer while you prepare the rice.
Cook the rice. In a separate 12-inch skillet, heat 2 tablespoons of olive oil over medium-low heat. Add the uncooked rice and cook, stirring, for 2-3 minutes. Then, season with ½ teaspoon salt and ½ teaspoon pepper. Add the chicken broth, diced tomatoes, and ½ cup marinara sauce. Stir to combine. Cover the skillet with a lid and simmer over low heat for 20 minutes. Lower the heat to the lowest setting and cook for 15 minutes. If the rice isn’t fully cooked after this time, add ¼ cup of broth and cook for a few more minutes until done.
Prep the peppers. While the rice cooks, prepare the bell peppers. Preheat the oven to 400°F. Cut the tops off the peppers and remove the core and thick membranes using a sharp knife. Then, brush the inside of each pepper with olive oil and sprinkle with ⅛ teaspoon each of salt and pepper. Place the peppers in a 9x13 pan, spacing them 1-2 inches apart. Roast the peppers for 20 minutes. Remove from the oven and let them cool.
Cook the meat. Lower the oven temperature to 350°F after the peppers have roasted. Heat 1 tablespoon of olive oil over medium heat in a cleaned skillet. Then, add the Italian sausage and ground beef, cooking until no longer pink. Next, add minced garlic, oregano, ¼ teaspoon of salt, and ¼ teaspoon of pepper. Cook for another 1-2 minutes, then drain any excess grease.
Mix the filling. Once the rice is done, combine it with the cooked meat mixture and stir in 1 cup of the marinara sauce. Add ½ cup of shredded mozzarella and ¼ cup of grated Parmesan to the rice and meat mixture, stirring until everything is well incorporated.
Assemble the peppers. Carefully remove the roasted peppers from the pan. Then, spread 1 cup of marinara sauce evenly across the bottom of the 9x13 pan. Return the peppers to the pan, and generously fill each pepper with the rice and meat mixture. Once filled, spoon about 2 tablespoons of marinara sauce over each stuffed pepper. Cover the pan with foil and bake at 350°F for 20 minutes.
Add the cheese and broil. After 20 minutes, remove the foil and sprinkle 1 tablespoon of Parmesan and 2 tablespoons of mozzarella over each stuffed pepper. Then, return the pan to the oven and bake for 10 minutes or until the cheese is melted. For a slightly browned top, switch the oven to broil and cook for 1-2 minutes, keeping a close eye to avoid burning.
Serve. Garnish the stuffed peppers with chopped parsley if desired, and serve warm.