Preheat oven to 375°F.
Cook quinoa in vegetable broth according to package directions until fluffy and liquid is absorbed.
Heat olive oil in a large skillet over medium heat. Sauté diced onion until soft and translucent, about 5 minutes.
Add minced garlic, cumin, chili powder, and garlic powder to the skillet. Cook for 1 minute until fragrant.
Stir in diced tomatoes, black beans, corn kernels, and cooked quinoa. Season with salt and pepper. Cook for 5 minutes, stirring occasionally.
Place pepper halves cut-side up in a baking dish. Add enough water to cover the bottom of the dish.
Fill each pepper half generously with the quinoa mixture. Top with shredded cheese if using.
Cover tightly with aluminum foil and bake for 30 minutes.
Remove foil and continue baking for 15-20 minutes until peppers are tender and cheese is golden brown.