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.
Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook until translucent, about 3-4 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet and cook until browned and cooked through, breaking it up with a spoon as it cooks, about 6-8 minutes.
Drain any excess fat from the skillet.
Stir in the cooked rice, diced tomatoes, tomato sauce, salt, black pepper, and Italian seasoning. Cook for 3-4 minutes until everything is well combined and heated through.
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 cheddar cheese over the tops of the stuffed peppers.
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes, until the cheese is melted and bubbly.
Remove from oven and let cool for 5 minutes before serving.