Cook the Ground Beef
Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish and set aside.
In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the chopped onion and garlic, cooking for about 3-4 minutes until softened.
Add the ground beef to the skillet, breaking it up with a spoon. Cook until the beef is browned and cooked through, about 5-7 minutes. Drain any excess fat if needed.
Prepare the Casserole Filling
To the cooked beef, add the diced bell peppers, cooked rice, diced tomatoes, and tomato paste (if using). Stir in the oregano, cumin, chili powder, salt, and pepper. Mix well to combine all the ingredients.
Assemble the Casserole
Pour the beef and rice mixture into the prepared baking dish and spread it out evenly.
Sprinkle the shredded cheddar cheese over the top of the casserole, covering it evenly.
Bake the Casserole
Cover the baking dish with aluminum foil and bake for 25-30 minutes. After 25-30 minutes, remove the foil and bake for an additional 10 minutes, or until the cheese is melted and bubbly.
Serve
Remove the casserole from the oven and let it rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.