1. Preheat oven to 375°F (190°C). Cut the tops off the bell peppers and remove the seeds and membranes. Place peppers in a baking dish.
2. In a skillet, heat olive oil over medium heat. Sauté onion and garlic until softened. Stir in cooked quinoa, marinara sauce, half the bacon, salt, and pepper. Mix well.
3. Stuff each pepper with the quinoa mixture, pressing it down gently. Top with shredded cheese and remaining crumbled bacon.
4. Bake for 20-25 minutes, or until the peppers are tender and the cheese is melted and golden.
5. Serve hot with extra marinara sauce on the side, if desired.