Prepare the Peppers: Preheat your oven to 375°F (190°C). Cut the tops off the sweet peppers and remove the seeds and membranes. Set them aside.
Cook the Quinoa: In a large saucepan, bring 2 cups of water to a boil. Add the quinoa, reduce the heat to low, and cover. Let it simmer for about 15 minutes, or until all the water is absorbed and the quinoa is tender.
Prepare the Filling: In a large skillet, heat the olive oil over medium heat. Add the diced tomatoes, corn, black beans, cumin, and paprika. Stir to combine and cook for about 5 minutes, allowing the flavors to meld.
Mix the Filling: Add the cooked quinoa to the skillet with the vegetable mixture and season with salt and pepper. Stir well to combine.
Stuff the Peppers: Carefully spoon the quinoa mixture into the hollowed-out sweet peppers. Pack the filling tightly but gently.
Bake: Place the stuffed peppers in a baking dish and cover with aluminum foil. Bake in the preheated oven for 30 minutes.
Finish: If using cheese, remove the foil after 30 minutes and sprinkle shredded cheese on top of each stuffed pepper. Return the peppers to the oven for an additional 5-10 minutes, or until the cheese is melted and bubbly.
Serve: Remove from the oven and garnish with fresh cilantro if desired. Serve hot.