Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Add the bell peppers and cook for 4 minutes to soften slightly. Remove and drain upside down on paper towels.
In a large skillet, heat olive oil over medium heat. Add the chopped onion and minced garlic and sauté until translucent, about 3-4 minutes.
Add the ground beef to the skillet and cook until browned, breaking it up with a spoon, about 6-8 minutes. Drain excess fat if necessary.
Stir in the cooked rice, diced tomatoes, dried oregano, dried basil, salt, and black pepper. Cook for 2 minutes to combine flavors. Remove from heat.
In a medium bowl, combine tomato sauce, tomato paste, sugar, garlic powder, onion powder, salt, and black pepper. Mix well.
Place the bell peppers upright in a baking dish. Spoon the beef and rice mixture evenly into each bell pepper, pressing gently to fill completely.
Pour the tomato sauce mixture evenly over and around the stuffed peppers in the baking dish.
Cover the baking dish with aluminum foil and bake for 35 minutes.
Remove the foil and sprinkle shredded mozzarella cheese on top of each stuffed pepper.
Return to the oven and bake uncovered for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Remove from oven and let cool for 5 minutes.
Garnish with chopped fresh parsley before serving.