Preheat your oven to 375°F (190°C).
Prepare the bell peppers by cutting off the tops and removing the seeds. Rinse them thoroughly.
In a large mixing bowl, combine the cooked quinoa or rice, black beans, corn kernels, diced tomatoes, diced onion, shredded cheese, ground cumin, chili powder, garlic powder, salt, and pepper. Mix well to combine all the ingredients.
Stuff the mixture into the hollowed-out bell peppers, pressing down gently to fill them evenly.
Place the stuffed bell peppers in a baking dish that's been lightly greased or lined with parchment paper.
Drizzle a bit of olive oil over the tops of the stuffed peppers to help them cook and brown in the oven.
Cover the baking dish with aluminum foil and place it in the preheated oven.
Bake the stuffed bell peppers for about 25-30 minutes, or until the peppers are tender and the filling is heated through.
Remove the foil and continue to bake for an additional 5-10 minutes, or until the cheese on top is melted and bubbly and the tops of the peppers are slightly browned.
Carefully remove the stuffed bell peppers from the oven.
Allow the stuffed bell peppers to cool slightly before serving.
Serve the vegetarian stuffed bell peppers on a plate and enjoy your hearty and flavorful meal!