Preheat oven to 375°F (190°C).
Bring a large pot of salted water to a boil. Add bell peppers and boil for 3–4 minutes to slightly soften. Drain and set aside.
In a large skillet over medium heat, heat olive oil. Add diced onion and cook for 2–3 minutes until softened.
Add ground beef and cook until browned, breaking it up with a spoon. Drain excess fat.
Stir in garlic, tomato paste, diced tomatoes, cooked rice, Italian seasoning, salt, and pepper. Let it cook for 2–3 minutes until well combined.
Remove from heat and mix in ½ cup shredded cheese.
Place the peppers upright in a baking dish. Spoon the beef and rice mixture into each pepper, pressing down slightly.
Top each stuffed pepper with remaining cheese.
Add a small amount of water (about ¼ cup) to the bottom of the baking dish to help steam the peppers. Cover with foil.
Bake for 30 minutes, then remove foil and bake uncovered for another 10–15 minutes, or until cheese is melted and bubbly.
Garnish with chopped parsley if desired, and serve warm.