Step 1: 1. Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish or a similar-sized casserole dish.

Step 2: 2. Place the bell pepper halves in the prepared baking dish, cut side up. Bake for 15-20 minutes, or until slightly softened.

Step 3: 3. While the bell peppers are baking, prepare the filling. In a large skillet, heat the olive oil over medium heat. Add the chopped onion, and cook for 3-4 minutes, until softened. Add the garlic, and cook for another 1 minute, until fragrant.

Step 4: 4. Stir in the cooked quinoa, black beans, corn, diced tomatoes, ground cumin, chili powder, smoked paprika, salt, and pepper. Cook for 5-7 minutes, until heated through. Remove the skillet from heat, and stir in 1 cup of the shredded cheese, if using.

Step 5: 5. Remove the bell peppers from the oven, and carefully stuff each pepper half with the quinoa and black bean mixture. Top the stuffed peppers with the remaining 1/2 cup of shredded cheese, if using.

Step 6: 6. Return the stuffed peppers to the oven, and bake for another 15-20 minutes, until the cheese is melted and the peppers are tender.

Step 7: 7. Remove from the oven, and allow the stuffed peppers to cool for 5 minutes before serving. Garnish with fresh chopped cilantro.