Preheat oven to 375°F (190°C). Lightly grease a baking dish.
Cut the tops off the bell peppers and remove the seeds and membranes. Set aside.
In a large skillet, heat olive oil over medium heat. Sauté onion and garlic until softened.
Add ground beef or turkey and cook until browned. Drain any excess fat.
Stir in diced tomatoes, cooked rice, oregano, cumin, salt, and pepper. Cook for another 5 minutes.
Remove from heat and mix in feta cheese and parsley.
Stuff each bell pepper with the filling and place them in the prepared baking dish.
Cover with foil and bake for 30 minutes.
Remove foil, sprinkle with mozzarella cheese if using, and bake for an additional 10 minutes until cheese is melted and peppers are tender.
Serve warm and enjoy!