01 - Preheat the oven to 375°F (190°C).
02 - Bring a large pot of salted water to a boil, then blanch the bell peppers for 3 minutes. Drain and set them upside down to dry.
03 - Heat olive oil in a large skillet over medium heat. Sauté the chopped onion for 3 to 4 minutes until softened. Add minced garlic and cook for 1 more minute.
04 - Add ground beef to the skillet, breaking it up and cooking until browned and fully cooked. Drain excess fat if necessary.
05 - Stir in drained diced tomatoes, cooked rice, half of the shredded cheese, parsley, oregano, salt, black pepper, paprika, and red pepper flakes if using. Cook for 2 to 3 minutes until heated through and thoroughly combined.
06 - Stand the blanched bell peppers upright in a baking dish and evenly spoon the beef and rice mixture into each one.
07 - Sprinkle the remaining shredded cheese over the filled peppers.
08 - Pour ¼ cup water into the bottom of the baking dish and cover loosely with aluminum foil.
09 - Bake covered for 30 minutes, then remove the foil and continue baking for 10 to 15 minutes until the cheese is golden and peppers are tender.
10 - Allow the stuffed peppers to cool for a few minutes before serving.