Preheat the oven to 375°F (190°C).
Cut the tops off the bell peppers and remove seeds.
In a bowl, mix cooked quinoa, black beans, corn, diced tomatoes, and spices.
Fill each bell pepper with the quinoa mixture.
Place stuffed peppers upright in a baking dish.
Drizzle with olive oil and season with salt and pepper.
Cover the dish with foil and bake for 25 minutes.
Remove foil and bake for an additional 10 minutes.
Garnish with chopped cilantro before serving.
Let cool for a few minutes before serving.