Preheat the oven to 375°F (190°C) and lightly spray a baking dish with nonstick spray.
Cut the tops off the bell peppers, remove seeds, and set aside.
In a skillet over medium heat, add olive oil and sauté chopped onions until translucent. Add ground meat, season with salt and pepper, and cook until browned.
Stir in cooked rice, diced tomatoes, minced garlic, and optional spices. Cook until heated through.
Stuff each bell pepper with the meat mixture, packing it gently.
Place stuffed peppers upright in the baking dish. Cover with foil and bake for 25-30 minutes. Remove foil, sprinkle cheese on top, and bake uncovered for an additional 10 minutes until bubbly.
