Preheat your oven to 375°F (190°C).
Cook your quinoa according to package instructions, about 15 minutes, and set aside.
In a skillet, heat olive oil over medium heat. Sauté diced onion and minced garlic for 2-3 minutes, then add ground chicken and cook until no longer pink, about 5-7 minutes. Season with Italian seasoning, salt, and pepper.
In a mixing bowl, combine cooked quinoa, sautéed chicken mixture, chopped spinach, and half of the mozzarella cheese. Mix well.
Prepare the bell peppers by cutting off the tops and removing seeds. Optionally blanch in boiling water for 5 minutes.
Stuff each pepper with the filling, pressing down gently, and top with remaining mozzarella cheese.
Cover the baking dish with foil and bake for 25 minutes. Remove foil and bake for an additional 10-15 minutes until cheese is bubbly and golden.
Let cool for a few minutes before serving. Enjoy!