Preheat oven to 375°F (190°C).

In a large skillet, brown the ground beef over medium heat. Drain off any excess grease.

Add the chopped onion and minced garlic to the skillet and cook until softened, about 5 minutes.

Stir in the cooked rice, tomato sauce, oregano, salt, and pepper. Mix well.

Stuff the bell pepper cavities evenly with the meat mixture.

Place the stuffed peppers in a baking dish. Add about 1/2 inch of water to the bottom of the dish.

Cover the baking dish tightly with foil.

Bake for 30 minutes. Remove the foil, sprinkle with shredded cheddar cheese, and bake for another 10 minutes, or until peppers are tender and cheese is melted and lightly browned.