Preheat the oven to 375°F (190°C). Prepare a baking dish by lightly greasing it or lining with parchment paper.

Prepare the peppers: Carefully cut the tops off the bell peppers and remove the seeds and membranes. If necessary, slightly trim the bottoms to make them stand upright. Place them in the prepared baking dish.

Cook the quinoa: In a medium saucepan, combine the rinsed quinoa and vegetable broth. Bring to a boil, then cover, reduce the heat, and simmer for 15-20 minutes, or until the broth is absorbed. Remove from the heat and fluff with a fork.

Mix the filling: In a large bowl, mix the cooked quinoa, black beans, corn, and chopped tomatoes. Season with cumin, paprika, garlic powder, salt, and pepper. Stir until everything is well combined.

Fill the peppers: Spoon the quinoa mixture into each bell pepper cavity, pressing down slightly to pack it in. Fill them up to the top.

Bake: Cover the dish with aluminum foil and bake in the preheated oven for about 30 minutes. Then, uncover and bake for an additional 10-15 minutes, or until the peppers are tender and the filling is heated through.

Serve: Remove the stuffed peppers from the oven. If desired, sprinkle with shredded cheese and return to the oven just until the cheese has melted. Garnish with optional toppings like fresh cilantro, sour cream, or avocado slices before serving.