Brush the pepper halves with half the oil, arrange them cut-side up in a baking dish and roast at 400°F for 15 minutes.
Heat the remaining oil in a skillet and cook the beef and onion over medium-high heat for 7 minutes until the beef reaches 160°F; drain excess fat.
Stir in the garlic, salt and oregano for 30 seconds, then add 1 cup tomato sauce and the cooked rice.
Fill the softened peppers evenly and spoon the remaining tomato sauce over the tops.
Cover and bake for 20 minutes, uncover, add the cheddar and bake for 8 minutes until melted.