Preheat the oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish.
In a medium saucepan, combine the 1 cup of rinsed quinoa and 2 cups of vegetable broth. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes or until liquid is absorbed. Fluff with a fork.
In a large mixing bowl, combine the cooked quinoa, 15 oz of black beans, 14.5 oz of diced tomatoes with their juices, 2 tbsp of taco seasoning, and 1/2 cup of the shredded cheddar cheese. Mix thoroughly.
Arrange the 4 halved bell peppers in the prepared baking dish, cut side up. Spoon the quinoa and black bean mixture generously into each pepper cavity.
Cover the baking dish tightly with aluminum foil and bake for 30 minutes, or until the peppers are tender.
Remove the foil, sprinkle the remaining 1/2 cup of cheddar cheese over the stuffed peppers, and bake for an additional 5 to 7 minutes until the cheese is melted and bubbly.
Serve warm, garnished with 1/4 cup of sour cream, 1 medium diced avocado, and 2 tbsp of freshly chopped cilantro.