Preheat oven to 375 degrees F. Cut bell peppers in half lengthwise and remove seeds and membranes. Place cut-side up in a baking dish in a single layer.

Heat a large skillet over medium heat. Add diced onion and cook for 3 to 4 minutes until softened and translucent. Add minced garlic and cook for 1 minute more, stirring constantly.

Add ground turkey to the skillet. Break apart with a spoon and cook for 5 to 7 minutes until fully browned with no pink remaining. Drain any excess fat.

Stir in cooked quinoa, black beans, corn, tomato sauce, chili powder, cumin, smoked paprika, oregano, salt, and pepper. Mix until fully combined and cook together for 2 minutes. Taste and adjust seasoning.

Spoon filling into each bell pepper half pressing down gently to pack it in. Filling should sit slightly mounded above the rim. Cover tightly with foil and bake for 25 to 30 minutes until peppers are tender when pierced with a fork.

Remove foil. If using cheese, sprinkle over each pepper and bake uncovered for 5 to 10 minutes until cheese is melted and lightly golden. Cool a few minutes before serving. Garnish with fresh cilantro or parsley if desired.