Preheat your oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds and membranes. Place them upright in a baking dish.
In a large skillet, heat the olive oil over medium heat. Add the red onion and garlic, sautéing until the onion is translucent, about 3-4 minutes.
Add the ground chicken to the skillet, breaking it apart with a spatula. Cook until browned and fully cooked, about 5-7 minutes.
Stir in the cooked quinoa, diced tomatoes, feta cheese, Kalamata olives, oregano, basil, salt, and pepper. Mix well and cook for an additional 2-3 minutes until heated through.
Spoon the chicken mixture into each bell pepper, packing it down gently.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, until the peppers are tender.
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.