Preheat the Oven: Preheat your oven to 375°F (190°C).
Prepare the Filling: In a large bowl, mix cooked quinoa, black beans, corn, diced tomatoes, cumin, salt, and pepper until well combined.
Stuff the Peppers: Stuff the bell peppers with the quinoa mixture, pressing down slightly to pack the filling. Place the stuffed peppers upright in a baking dish.
Bake: Cover the baking dish with aluminum foil and bake for 25-30 minutes. If using cheese, remove the foil after 20 minutes, sprinkle cheese on top of each pepper, and bake uncovered for the remaining time until the cheese is melted and bubbly.
Garnish and Serve: Remove from the oven and let cool slightly. Garnish with fresh cilantro before serving.