Preheat oven to 375°F (190°C). Drizzle peppers with a little oil and place upright in a baking dish.
Heat oil in a skillet over medium; sauté onion until soft, 3–4 minutes. Add garlic and cook 30 seconds.
Add minced chicken, breaking up with a spoon; cook until no pink remains, about 6–8 minutes. Season with chili powder, cumin, salt, and pepper.
Stir in cooked quinoa, diced tomatoes, and corn; heat through and adjust seasoning.
Spoon mixture into peppers, top with cheese, cover with foil, and bake 25–30 minutes until peppers are tender. Remove foil last 5 minutes to brown cheese.