Cut off the tops of the bell peppers and remove the seeds and membranes. Set aside.
Heat the olive oil in a large skillet over medium-high heat. Add the diced onion and cook until soft, about 4-5 minutes.
Add the ground beef. Cook until browned, breaking up clumps with a wooden spoon, about 8-10 minutes. Drain off excess fat if needed.
Stir in the diced tomatoes, tomato sauce, cooked rice, Italian seasoning, garlic powder, salt, and pepper. Cook until heated through, about another 3-5 minutes.
Spoon the beef and rice mixture into the prepared peppers and packing gently with the back of the spoon.
If needed, coat the air fryer basket lightly with nonstick cooking spray or an oil mister. Preheat to 360°F for 3-5 minutes.
Arrange the peppers in the air fryer basket with a little space between each. Air fry for 12–15 minutes, or until peppers are tender and filling is hot.
Sprinkle the shredded cheese on top and air fry for another 2–4 minutes, until melted and bubbly.
Let peppers rest for 2–3 minutes before serving.