Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with cooking spray or butter.
Cook the Ground Beef: In a large skillet, heat olive oil over medium heat. Add ground beef and cook until browned. Drain any excess fat and set aside.
Sauté the Vegetables: Add chopped onions to the skillet and sauté for 3-4 minutes until soft. Add minced garlic and cook for another minute.
Mix in Rice and Sauce: Stir in cooked rice, tomato sauce, oregano, basil, salt, and pepper. Simmer for 3-4 minutes to let the flavors combine.
Assemble the Casserole: Spread the meat mixture evenly in the baking dish. Slice bell peppers in half, remove seeds, and place them on top of the meat mixture.
Add Cheese: Sprinkle shredded cheddar cheese evenly over the top.
Bake: Cover with foil and bake for 25-30 minutes, until peppers are tender. Remove the foil and bake for an additional 5-10 minutes until the cheese is golden and bubbly.
Rest and Serve: Let the casserole rest for 5-10 minutes before serving. Enjoy!