Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Add the bell peppers and cook for 5 minutes to soften slightly. Remove and drain well.
In a large skillet, heat olive oil over medium heat. Add chopped onion and cook until translucent, about 3-4 minutes.
Add minced garlic and cook for 1 minute until fragrant.
Add ground beef to the skillet and cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes. Drain excess fat if necessary.
Stir in cooked rice, drained diced tomatoes, tomato sauce, salt, black pepper, and Italian seasoning. Cook for 3-4 minutes until heated through and combined.
Remove the skillet from heat and stir in half of the shredded cheddar cheese.
Place the bell peppers upright in a baking dish. Spoon the beef and rice mixture evenly into each bell pepper, filling them completely.
Sprinkle the remaining shredded cheddar cheese on top of each stuffed pepper.
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 and the peppers are tender.
Remove from oven and let cool for 5 minutes before serving.