Gather all ingredients.

Bring rice and water to a boil in a saucepan over high heat. Reduce heat to medium-low, cover, and simmer until the rice is tender, and the liquid has been absorbed, 20 to 25 minutes. Set the cooked rice aside.

Meanwhile, preheat the oven to 375 degrees F (190 degrees C).

To make the sauce: Cook onion and olive oil over medium heat until onion begins to soften, about 5 minutes. Transfer 1/2 of the cooked onion to a large bowl and set aside.

Stir marinara sauce, beef broth, balsamic vinegar, and red pepper flakes into the skillet with cooked onion; cook and stir for 1 minute.

Pour sauce mixture into a 9x13-inch baking dish and set aside.

Combine ground beef, Italian sausage, diced tomatoes, Italian parsley, garlic, salt, black pepper, and cayenne pepper into the bowl with reserved onions; mix well. Stir in cooked rice and one cup of Parmigiano Reggiano. Stuff bell peppers with beef and sausage mixture.

Place stuffed bell pepper halves in the baking dish over tomato sauce; optionally sprinkle with additional Parmigiano Reggiano, cover baking dish with aluminum foil, and bake in the preheated oven for 45 minutes.

Remove aluminum foil and continue to bake until meat is no longer pink, bell peppers are tender and cheese is browned on top, an additional 20 to 25 minutes. Serve hot and enjoy!