Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a baking dish or line it with foil.

Prepare the Bell Peppers: Cut the tops off the bell peppers and remove the seeds and membranes. Set the peppers aside. If needed, slice a small piece off the bottom of each pepper so they stand upright in the dish.

Cook the Filling: Heat olive oil in a skillet over medium heat. Add the chopped onion and garlic and sauté until softened, about 2-3 minutes. Add the ground beef and cook until browned, breaking it apart as it cooks. Drain excess fat if necessary.

Mix the Stuffing: In a large bowl, combine the cooked rice, cooked beef mixture, tomato sauce, oregano, salt, and pepper. Stir until evenly mixed.

Stuff the Peppers: Fill each pepper generously with the meat and rice mixture. Top each pepper with a little extra tomato sauce.

Bake: Place the stuffed peppers in the prepared baking dish. Cover the dish with foil and bake for 30 minutes.

Add Cheese: After 30 minutes, remove the foil and sprinkle the tops of the peppers with shredded cheese. Return the dish to the oven and bake uncovered for another 10-15 minutes, or until the cheese is melted and bubbly.

Serve: Let the stuffed peppers cool for a few minutes before serving. Garnish with fresh parsley if desired.