Preheat the oven to 350 degrees F. Grease a 9x13 (3-quart) baking dish and set aside.
Prepare the quinoa according to package instructions if you haven't made it in advance. It usually takes about 15 minutes to cook, so get it going while you prep the rest of the ingredients.
Warm the olive oil in a medium skillet over medium heat. Add the diced pepper and onion, and cook for 5 to 7 minutes until softened. Stir in the chili powder, cumin, and oregano, and cook for a few more minutes. Remove from heat.
Transfer the peppers and onions to a large mixing bowl. Add the cooked quinoa, black beans, corn, diced tomatoes, and green chiles. Stir well. Season with salt and pepper to taste.
Spread the quinoa mixture into the prepared baking dish. Sprinkle the shredded cheese on top.
Bake the casserole for 20 to 25 minutes until bubbly. Add toppings of choice before serving. Enjoy!
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