Preheat your oven to 375°F (190°C). Grease a baking dish large enough to hold the halved bell peppers.
In a large skillet, heat some olive oil over medium heat. Add the diced onion and cook until softened, about 5 minutes.
Add the minced garlic, ground cumin, chili powder, paprika, salt, and pepper.
Cook for another 1-2 minutes until fragrant.
Add the cooked quinoa or brown rice, black beans, corn kernels, and diced tomatoes to the skillet.
Stir everything together and cook for a few more minutes until heated through.
Taste and adjust seasoning if needed.
Arrange the halved bell peppers in the prepared baking dish, cut side up.
Spoon the quinoa and bean mixture evenly into each pepper half, pressing down gently to pack the filling.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25-30 minutes, or until the peppers are tender.
Remove the foil from the baking dish and sprinkle the shredded cheese evenly over the stuffed peppers.
Return the dish to the oven and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly.
Once done, remove the stuffed bell peppers from the oven and let them cool slightly before serving.
Serve the stuffed bell peppers hot, garnished with chopped fresh cilantro, sliced avocado, and a dollop of Greek yogurt or sour cream if desired.