Preheat your oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds.
In a large mixing bowl, combine cooked quinoa, black beans, corn, and diced tomatoes.
Add cumin, chili powder, salt, and pepper to the mixture.
Stuff each bell pepper with the quinoa mixture, pressing down gently.
Place stuffed peppers upright in a baking dish.
Cover the dish with aluminum foil.
Bake for 20 minutes, then remove the foil and bake for an additional 10 minutes.
Once cooked, remove from oven and let cool slightly.
Garnish with fresh cilantro before serving.