Prepare the Peppers:
Preheat the oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds and membranes. Set the peppers aside.
If desired, lightly steam the peppers in a pot of boiling water for 5 minutes to soften them before stuffing (optional, but can help them cook more evenly).
Prepare the Filling:
In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the chopped onion and garlic, and sauté for 2-3 minutes until softened and fragrant.
Add the ground meat to the skillet and cook, breaking it up with a spoon, until browned and cooked through. Drain any excess fat.
Stir in the cooked rice, diced tomatoes, oregano, cumin, salt, and pepper. Cook for another 2-3 minutes to combine the flavors.
Stuff the Peppers:
Carefully spoon the filling into each bell pepper, packing it in tightly. Place the stuffed peppers upright in a baking dish.
Make the Tomato Sauce (optional):
In a small bowl, combine the tomato sauce, olive oil, dried basil, garlic powder, salt, and pepper. Pour the sauce over the stuffed peppers.
Bake the Peppers:
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil, sprinkle the tops of the stuffed peppers with shredded cheese, and return them to the oven. Bake uncovered for an additional 10-15 minutes, or until the cheese is melted and bubbly and the peppers are tender.
Serve:
Serve the stuffed peppers hot, garnished with fresh herbs or extra cheese if desired.