Preheat your oven to 350 degrees. Slice your peppers crosswise closer to the top of the pepper. Remove the seeds and ribs and discard. Chop up the pepper tops and set aside. Place the cut peppers into a baking dish and set aside.
Heat one tablespoon of olive oil in a large sauté pan or skillet over medium high heat, and after a few minutes, add the onion and cook until translucent and softened. Add in the chopped peppers (from the tops) and sauté for about 3 to 4 minutes adding one more tablespoon of olive oil if you need it. Add the garlic and sauté for 30 seconds. Move your vegetable mixture to the outer edges of the pan and add in the lean ground beef or turkey into the center. Cook until all browned and there's no more pink.
When fully cooked through, combine the vegetable mixture with the cooked ground beef or turkey and season with salt and pepper. Add the cooked brown rice, and your marinara sauce. Stir to combine and let cook until everything is heated through. Taste and season with more salt and pepper if desired.
Stuff the peppers with the meat and rice filling, top with some shredded mozzarella cheese (if using) and bake in the oven for 20 to 25 minutes*, or until the peppers are tender and the cheese is starting to brown slightly. Garnish with fresh parsley and serve while hot!
