Prepare the Peppers: Preheat the oven to 375°F (190°C). Cut the tops off the bell peppers and remove the seeds and membranes.
Cook the Filling: In a large skillet, cook the ground beef or turkey over medium heat until browned. Add the diced onion and minced garlic, and cook until the onion is translucent. Stir in the cooked quinoa, marinara sauce, Italian seasoning, salt, and black pepper.
Stuff the Peppers: Fill each bell pepper with the meat mixture. Place the stuffed peppers in a baking dish.
Bake: Cover the dish with foil and bake for 30 minutes. Remove the foil, sprinkle the shredded mozzarella cheese on top of each pepper, and bake for an additional 10-15 minutes until the cheese is melted and bubbly.