Preheat the oven: Preheat your oven to 190°C (375°F).
Cook quinoa: In a medium pot, combine quinoa and vegetable broth. Bring to a boil, then reduce heat, cover, and simmer for about 15 minutes until quinoa is fluffy and broth is absorbed. Remove from heat and let sit covered for 5 minutes.
Prepare peppers: While quinoa cooks, cut the tops off the bell peppers and remove the seeds. Stand the peppers upright in a baking dish.
Mix filling: In a large bowl, combine cooked quinoa, black beans, corn, diced tomatoes, cumin, chili powder, smoked paprika, garlic powder, and half of the shredded cheese. Season with salt and pepper to taste.
Stuff peppers: Carefully spoon the quinoa mixture into the hollowed-out peppers and press down lightly. Top the stuffed peppers with the remaining shredded cheese.
Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then remove the foil and bake for an additional 10-15 minutes until the cheese is melted and bubbly.
Garnish and serve: After baking, let the stuffed peppers cool for a few minutes. Garnish with freshly chopped cilantro and serve with avocado slices if desired.