Preheat oven to 375°F. Halve the bell peppers and remove the seeds and membranes. Place the peppers in a 13x9 inch baking dish, cut side up. Pour ¼ cup of water into the bottom of the baking dish and cover with foil. Bake for 20 minutes, or until the peppers are tender.
Meanwhile, place the rice and 1 cup water in a small saucepan over medium high heat. Bring to a boil. Reduce heat to low, cover, and cook according to package directions (my white rice took 20 minutes). Let rice sit for 5 minutes after cooking, then uncover and fluff with a fork.
While the rice is cooking, heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook for 2–3 minutes, until the onion begins to turn translucent. Add the ground sirloin, and cook, breaking up with a wooden spoon, for 5–6 minutes until no longer pink. Stir in the garlic cloves, oregano, basil, and ¾ teaspoon salt. Cook for 30 seconds longer.
Add ½ cup of the mozzarella, tomato sauce, and cooked rice to the meat mixture. Stir to combine.
When the peppers are tender, remove them from the oven. Carefully lift the foil—there will be steam! Remove the peppers from the pan and pour out the water. Pour out any water that has collected in the peppers as well (I use tongs for this as the peppers are still hot).
Season the inside of the peppers with the remaining ¼ teaspoon salt. Divide the meat and rice mixture evenly among the peppers. Top with the remaining 1 cup cheese.
Bake for 5–7 minutes, or until the cheese has melted and the peppers are warm throughout. Garnish with parsley, if desired, and serve immediately.
Notes
To make these peppers vegetarian, substitute one can of rinsed and drained cannellini beans in place of the ground beef. There's no need to cook the beans—just add them in with the rice and tomato sauce. You can substitute 1 lb of ground turkey for the ground beef.Reheat the peppers in the microwave or in the oven (375°F, covered, for about 30 minutes if cold from the refrigerator).