Preheat the 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 diced red onion and garlic, sautéing until the onion is translucent, about 3-4 minutes.
Add the ground chicken to the skillet, cooking until browned and cooked through, about 5-7 minutes.
Stir in the cooked quinoa, diced tomatoes, feta cheese, Kalamata olives, oregano, basil, salt, and black pepper. Mix until well combined and heated through.
Spoon the chicken mixture evenly into each bell pepper, packing it down slightly.
Drizzle lemon juice over the stuffed peppers and cover the baking dish with foil.
Bake in the preheated oven for 30 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 if desired before serving.