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

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

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.

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.

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.

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

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