Sliced the top ½ inch from the tops of the peppers and cut the stems from the tops. Cut the core from the inside of the peppers and strip away any remaining seeds. Cut away a thin slice from the bottom of the peppers so that they peppers can stand upright on their own. Poke 3-4 small holes in the bottom of each pepper so the juices can drain out during cooking.
Pour 2 ½ cups tomato sauce into a 9x13 inch baking dish. Add onion, beef broth and red pepper flakes; spread mixture out evenly over bottom. Stand the peppers upright in the dish.
Combine ground beef, cooked rice, cheese, ¼ cup of the parsley 2 tablespoons of tomato sauce, garlic, salt and black pepper into a large mixing bowl.
Lightly stuff each pepper with the meat mixture. Spread 1 tablespoon of the remaining tomato sauce on the top of each stuffed pepper and place the pepper tops back on top.
Lightly cover the peppers with a piece of parchment paper and cover the dish tightly in foil. Lay the casserole dish on a baking sheet.
Bake 1 hour. Remove foil and parchment paper and bake for another 20-30 minutes.
Sprinkle each pepper with ½ teaspoon parsley and drizzle with a spoonful of pan juices.