Preheat your oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds and membranes. Place the peppers upright in a baking dish.
In a medium saucepan, combine the quinoa and vegetable broth. Bring to a boil, then reduce heat to low, cover, and simmer for about 15 minutes until the quinoa is fluffy and the liquid is absorbed.
In a large mixing bowl, combine the cooked quinoa, chopped onion, minced garlic, corn, black beans, cumin, chili powder, salt, black pepper, and diced tomatoes. Mix well until all ingredients are combined.
Stuff each bell pepper with the quinoa mixture, packing it in well. Top the stuffed peppers with shredded cheese.
Cover the baking dish with aluminum foil (optional) and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool slightly. Garnish with fresh cilantro or parsley if desired.