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1
Preheat the oven to 375 degrees F (190 degrees C).
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2
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.
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3
Cook onion and olive oil over medium heat until onion begins to soften, about 5 minutes. Transfer half of cooked onion to a large bowl and set aside.
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4
Stir marinara sauce, beef broth, balsamic vinegar, and red pepper flakes into the skillet; cook and stir for 1 minute.
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5
Pour sauce mixture into a 9x13-inch baking dish and set aside.
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6
Combine ground beef, Italian sausage, diced tomatoes, Italian parsley, garlic, salt, black pepper, and cayenne pepper into bowl with reserved onions; mix well. Stir in cooked rice and Parmigiano Reggiano. Stuff green bell peppers with beef and sausage mixture.
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7
Place stuffed green bell pepper halves in the baking dish over tomato sauce; sprinkle with remaining Parmigiano-Reggiano, cover baking dish with aluminum foil, and bake in the preheated oven for 45 minutes.
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8
Remove aluminum foil and bake until the meat is no longer pink, the green peppers are tender and the cheese is browned on top, an addition 20 to 25 minutes.
