01 - Preheat the oven to 375°F (190°C) and arrange a rack in the center.
02 - In a large bowl, mix cooked quinoa, black beans, corn, diced tomatoes, ground cumin, chili powder, salt, and pepper until well blended.
03 - Fill each bell pepper evenly with the quinoa-vegetable mixture, packing gently but not overfilling.
04 - Sprinkle shredded cheese over the filled peppers, if using.
05 - Place stuffed peppers upright in a baking dish. Bake for 30 to 35 minutes, until the peppers are tender and the filling is heated through.