Heat oven to 375°F (190°C); line a baking sheet with parchment or lightly grease a dish.
Cut tops off peppers; remove seeds and membranes and set aside.
Heat oil in a large skillet over medium; cook onion and garlic 3-5 minutes until softened.
Stir in shredded chicken.
Stir in pesto and optional cooked quinoa; heat through.
Season with salt and pepper.
Fill peppers, pressing mixture down lightly.
Arrange in a single layer on prepared sheet or dish.
Bake 25-30 minutes until peppers are tender and edges lightly charred.
Cool a few minutes before serving.