In a large skillet, cook the onion and olive oil over medium heat until softened.
Add in the sauce, broth, vinegar, and red pepper flakes, stirring to combine, and cooking for an additional minute.
Transfer the sauce mixture to the bottom of a 9x13" baking dish. Use a spoon to spread out evenly.
To a large mixing bowl, add the beef, sausage, tomatoes, parsley, garlic, salt, black pepper, and use your hands to really mix everything together well. Stir in the rice and cheese.
Stuff the peppers with the meat & rice mixture.
Place stuffed green bell pepper halves in the baking dish over tomato sauce; sprinkle with additional cheese, cover baking dish with aluminum foil, and bake at 375 degrees for 45 minutes.
Remove the foil and return the dish to the oven, baking adn additional 20-25 minutes.