Preheat the oven to 375°F (190°C).
Rinse the quinoa under cold water. In a medium saucepan, combine quinoa and vegetable broth. Bring to a boil, then reduce heat to low, cover, and simmer for about 15 minutes or until liquid is absorbed and quinoa is fluffy.
While the quinoa is cooking, prepare the peppers by cutting the tops off the bell peppers and removing the seeds and membranes. Arrange the peppers upright in a baking dish.
In a mixing bowl, combine the cooked quinoa, black beans, corn, diced onion, minced garlic, cumin, paprika, salt, black pepper, and diced tomatoes. Mix well until combined.
Stuff each bell pepper with the quinoa mixture, pressing it down gently to pack it in.
If desired, sprinkle shredded cheese on top of each stuffed pepper.
Add a little water to the bottom of the baking dish to help steam the peppers while they cook. Cover the dish with aluminum foil.
Bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 5 minutes, or until the peppers are tender and the cheese is melted.
Remove from the oven, garnish with fresh cilantro or parsley if desired, and serve hot.