Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Add the bell peppers and cook for 3 minutes to soften slightly. Remove and drain upside down on paper towels.
Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet. Cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes. Drain excess fat if necessary.
Remove the skillet from heat. Stir in the cooked rice, diced tomatoes, shredded cheddar cheese, chopped parsley, dried oregano, salt, and black pepper until well combined.
Place the bell peppers upright in a baking dish. Spoon the beef and rice mixture evenly into each pepper, packing gently.
Sprinkle the shredded mozzarella cheese evenly over the tops of the stuffed peppers.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes until the cheese is melted and bubbly.
Remove from the oven and let cool for 5 minutes before serving.