Cut the tops off the bell peppers and remove the seeds and membranes. Arrange them upright in a baking dish.
In a medium pot, combine the quinoa and vegetable broth. Bring to a boil, then reduce heat to low, cover, and simmer for about 15 minutes or until quinoa is fluffy and the broth is absorbed.
While the quinoa cooks, heat olive oil in a large skillet over medium heat. Sauté the chopped onion and minced garlic until the onion becomes translucent, about 3-4 minutes.
Add the diced tomatoes, black beans, corn, cumin, smoked paprika, and chili powder (if using) to the skillet. Stir well and cook for about 5 minutes, allowing the flavors to meld. Season with salt and pepper to taste.
Once the quinoa is cooked, fluff it with a fork and mix it into the vegetable mixture in the skillet. Stir until evenly combined.
Stuff each bell pepper with the quinoa and vegetable filling, pressing down gently to pack it in.
Pour a little vegetable broth (or water) into the bottom of the baking dish to keep the peppers moist while baking.
Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the peppers are tender.
Remove from the oven, let cool slightly, and garnish with fresh cilantro and avocado slices before serving.
Notes
Feel free to customize the filling with your favorite vegetables.