Preheat oven to 375°F (190°C).
- In a skillet, heat olive oil over medium heat. Add diced onion and garlic; sauté until softened, about 3-4 minutes.
- Add ground beef, breaking it apart, and cook until browned and cooked through. Drain excess fat.
- Stir in cooked brown rice, diced tomatoes, oregano, cumin, salt, and pepper. Cook for another 2-3 minutes until heated through.
- Stuff each bell pepper with the beef and rice mixture, pressing gently to pack filling. Place peppers upright in a baking dish.
- If using, sprinkle cheese on top of each stuffed pepper.
- Cover with foil and bake for 30 minutes. Remove foil and bake an additional 10-15 minutes until peppers are tender and cheese is bubbly and golden.
- Let cool slightly before serving.