Preheat the oven to 350˚F. Place the peppers cut side up in a 9 by 13-inch baking dish and bake for 10 minutes. Remove and set aside.
Meanwhile, in a large saute pan, heat the olive oil over medium heat. Add the onion, celery and carrots, and cook, stirring often, for 5 minutes, or until the vegetables start to soften.
Add the turkey and season with salt and pepper. Cook, breaking up the turkey into small chunks for 5 to 7 minutes or until cooked through. Add the garlic, Italian seasoning and red pepper flakes and cook for 1 minute.
Add diced tomatoes and cooked quinoa and stir until well combined. Remove from heat. Using a serving spoon or cookie scoop, divide the turkey and quinoa mixture evenly between the peppers.
Bake for 15 minutes, then sprinkle the cheese evenly on top of each pepper and cook for an additional 15 minutes, or until melted and bubbly. Let cool slightly before serving.