Preheat your oven to 400°F (200°C). Begin by preparing the bell peppers: slice the tops off and remove the seeds and membranes. Set them aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.
Add the ground turkey to the skillet, breaking it up with a spoon. Cook until browned, about 6-8 minutes. Drain any excess fat if necessary.
Stir in the cooked quinoa, diced tomatoes, chili powder, cumin, paprika, salt, and pepper. Mix well and let the mixture simmer for about 5 minutes to allow the flavors to meld together.
Spoon the turkey and quinoa mixture into each bell pepper, packing it down gently. Place the stuffed peppers upright in a baking dish. Cover the dish with aluminum foil.
Bake in the preheated oven for 30 minutes. After 30 minutes, remove the foil, sprinkle cheese on top of each pepper, and return to the oven for an additional 10 minutes, or until the cheese is melted and bubbly.
Once done, remove from the oven and let cool for a few minutes before serving. Enjoy your ground turkey stuffed peppers!