Gather all ingredients. Preheat the oven to 375 degrees F (190 degrees C).

Slice peppers ½-inch from the top; cut out stems and set aside to use as tops. Cut core from the inside of peppers and strip away any seeds. Cut off a thin slice from the bottoms so peppers can stand upright. Poke about 4 tiny holes in the bottoms to let juices drain.

Pour 2 ½ cups tomato sauce into a 9x13-inch baking dish. Add onion, beef broth, and red pepper flakes; spread out mixture evenly over the bottom.

Set bell peppers upright in the dish.

Combine ground beef, cooked rice, Parmigiano-Reggiano cheese, 2 tablespoons tomato sauce, ¼ cup parsley, garlic, salt, and black pepper in a large mixing bowl.

Lightly stuff peppers with meat mixture. Spread 1 tablespoon tomato sauce on top of each portion of stuffing; place reserved tops onto peppers. Lay a piece of parchment paper loosely on top of peppers and cover the dish tightly with foil. Place the dish on top of a baking sheet.

Bake in the preheated oven until peppers have softened slightly, about 1 hour. Remove foil and parchment paper. Continue baking until meat filling is cooked through and peppers are soft, sweet, and tender, 20 to 30 more minutes.

Sprinkle each pepper with ½ teaspoon parsley and drizzle with a spoonful of pan juices.