Bring a large saucepan of water to a boil over highheat. Salt generously. Add the pasta shells and cook according to packagedirections, until al dente. Do not overcook. Drain and rinse with cold water.
Heat the olive oil in a large nonstick skillet set over medium heat. Add the onion and cook, stirring occasionally, until the onion is tender, 5 to 6 minutes. Add the garlic and cook for 1 minute.
Increase the heat to medium-high. Add the ground turkey to the skillet, and season with the oregano, salt and pepper. Cook, breaking up turkey with a wooden spoon, until the turkey is cooked through. Stir in the spinach and remove from the heat.
Place the cottage cheese in a food processor and puree until smooth, scraping down the sides as needed.
Transfer the cottage cheese to a large bowl and stir in the pesto. Add the turkey mixture. Stir until combined with the pesto cottage cheese.
Fill each shell with 2 tablespoons of the turkey mixture.
Preheat the oven to 350 degrees F. Coat a 9- by 13-inch baking dish with cooking spray, then spread ¾ cup of the marinara sauce on the bottom of the dish.
Arrange the stuffed pasta shells, open side up, in the baking dish. Spread the remaining 1 ½ cup marinara sauce over the shells and sprinkle with the Parmesan cheese.
Cover with foil and bake until the marinara sauce is bubbling, about 25 minutes. Remove the foil and bake until the Parmesan cheese is melted, about 5 minutes. Sprinkle with parsley, if desired. Serve.