Preheat the oven to 375°F. Cut the tops off the peppers, remove the seeds and membranes, and lightly sprinkle the insides with 1/4 tsp salt. Arrange upright in a 9x13-inch baking dish.
Heat the olive oil in a skillet over medium heat. Add the onion and sauté 3-4 minutes until softened. Add the garlic and ground turkey and cook until browned, 6-8 minutes. Drain if needed.
Stir the cooked quinoa, diced tomatoes, oregano, basil, 1/2 tsp salt, and 1/2 tsp pepper into the skillet. Mix well and simmer 2-3 minutes to combine.
Fill each pepper generously with the mixture and top with mozzarella.
Cover the dish loosely with foil and bake 30 minutes. Remove the foil and bake 10 minutes more, until the cheese is melted and bubbly.
Garnish with fresh parsley or basil before serving.