Cut the peppers in half (longways) removing the seeds and white ribs. Place them in a pot of boiling water and parboil for about 5 minutes. Drain.
Cook the rice (or use leftover cooked rice). Meanwhile, on medium heat, cook the ground meat, onions, mushrooms and garlic (cook the last 30 seconds so it doesn't burn) together. Once that is done, combine the rest of the ingredients with the rice and meat mixture. Mix thoroughly. Simmer for 10 minutes.
Place the pepper halves in a buttered baking dish. Spoon the filling into each pepper. Top with the freshly grated cheese. It's ok if you have extra filling fall to the bottom of the dish. If using the tomato sauce or strained tomatoes, add it to the bottom of the dish at this time.
Bake at 350°F for about 20-25 minutes or until the cheese is melted and the filling is hot and cooked through.
Top with chopped fresh parsley and serve.