Preheat your oven to 375°F (190°C). Cut the tops off the bell peppers and remove the seeds and membranes. If needed, slice a thin piece off the bottom to help them stand upright.
Rinse the quinoa thoroughly under cold water. Combine with vegetable broth in a medium saucepan. Bring to a boil, reduce heat, cover, and simmer for 15-20 minutes until liquid is absorbed.
While the quinoa cooks, heat olive oil in a large skillet over medium heat. Sauté onions until translucent (about 5 minutes), then add garlic and cook for another minute.
Add the cooked quinoa, black beans, corn, and tomatoes to the skillet. Season with cumin, paprika, oregano, salt, and pepper. Stir to combine.
Fill each pepper with the quinoa mixture. Place them in a baking dish and add about 1/4 cup water to the bottom of the dish.
Cover with foil and bake for 30-35 minutes. If using cheese, remove foil, top with cheese, and bake for an additional 5 minutes until cheese melts.