Preheat your oven to 375°F (190°C). Slice the tops off the bell peppers and remove the seeds and membranes. Place them upright in a baking dish.
In a medium saucepan, bring the vegetable broth to a boil. Add the rinsed quinoa, reduce heat to low, cover, and simmer for about 15 minutes until the quinoa is fluffy and liquid is absorbed.
In a skillet over medium heat, sauté the chopped red onion and minced garlic until translucent, about 3-4 minutes.
Stir in the black beans, corn, cumin, smoked paprika, salt, and pepper. Cook for another 2 minutes to heat through.
Mix the cooked quinoa into the vegetable mixture until well combined.
Spoon the quinoa mixture into each bell pepper, packing them gently.
Cover the baking dish with foil and bake for 30 minutes. Remove the foil and bake for an additional 10 minutes to lightly char the tops.
Let the stuffed peppers cool slightly before serving. Garnish each pepper with fresh cilantro and avocado slices.
Notes
Feel free to customize the filling with your favorite vegetables.