Preheat your oven to 375°F.
Cut the tops off the peppers and remove the seeds and membranes.
Heat the olive oil in a large skillet over medium-high heat. Add the onion and garlic and sauté for 2-3 minutes, until softened.
Add the cumin and chili powder and cook for another minute.
Add the black beans, diced tomatoes, quinoa, and corn to the skillet. Stir to combine and season with salt and pepper to taste.
Spoon the mixture into the hollowed-out peppers, packing it down tightly.
Place the stuffed peppers in a baking dish and bake for 25-30 minutes, until the peppers are tender.
If using cheese, sprinkle it over the top of the peppers during the last 5 minutes of baking.
Remove from the oven and let cool for a few minutes before serving.