Preheat your oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds and membranes. Set aside.
Rinse the quinoa under cold water. In a medium saucepan, combine the quinoa and vegetable broth. Bring to a boil, then reduce the heat to low and cover. Cook for about 15 minutes or until the quinoa is fluffy and the liquid is absorbed. Remove from heat and set aside.
In a large skillet, combine the black beans, corn, diced tomatoes, cumin, chili powder, salt, and pepper. Heat over medium heat until warmed through, about 5-7 minutes.
Stir the cooked quinoa into the skillet with the bean mixture until well combined. If using cheese, stir in half of it now.
Spoon the quinoa and vegetable mixture into the hollowed peppers, packing it lightly. If desired, top with the remaining cheese.
Place the stuffed peppers in a baking dish and cover with foil. Bake in the preheated oven for 25-30 minutes, until the peppers are tender.
Remove the foil and bake for an additional 5 minutes to melt the cheese on top, if using.
Allow to cool slightly before serving. Garnish with fresh cilantro, if desired.