Bring rice and water to a boil in a medium pot. Cover pot and reduce heat to medium-low Simmer until the liquid has been absorbed - about 20 to 25 minutes.
Cook onion and olive oil over medium heat until onion begins to soften, about 10 minutes. Take half of cooked onion and place in a large bowl for later use.
Add the marinara sauce, beef broth, balsamic vinegar, and red pepper flakes to the pot with the cooked onion. Continue to cook and stir for an additional minute.
Pour sauce mixture into a 9x13-inch baking pan.
Combine ground beef, hot sausage, tomatoes, parsley, garlic, salt, black pepper, and cayenne pepper(Optional) into a bowl with leftover onions. Mix all ingredients well. Add cooked rice and 1 cup of the parmesan cheese.
Stuff green bell peppers with beef and sausage mixture.
Place stuffed green bell pepper halves in the baking dish with tomato sauce. Sprinkle the peppers with the remaining 1/2 cup of parmesan cheese. Cover baking dish with aluminum foil and bake in the oven for 45 minutes at 375 degrees.
Remove aluminum foil and bake and additional 30 min until the meat juice is no longer pink, the peppers are tender and the cheese is slightly brown on top.