In a medium skillet, heat olive oil over medium-high heat. Once the oil is hot, add onion, ground turkey and Italian seasoning. Cook meat, breaking up into small pieces until no longer pink, about 5-7 minutes. Remove from heat, drain and let cool.
Cook manicotti noodles according to package instructions, making sure to cook them al dente. Once they’re done, drain water and add back to the pot. Drizzle olive oil over cooked noodles (this will prevent them from sticking together); set aside.
Preheat oven to 350 degrees F and spray a 9×13 baking dish with non-stick cooking spray; set aside.
While the noodles are cooking, add the ricotta, 1 1/2 cups mozzarella cheese, 1/2 cup Parmesan cheese, and parsley to a large bowl, mix well. Add garlic, salt, pepper and cooled meat into the cheese mixture and mix well.
Spoon 1 cup of the marinara sauce over the bottom of the prepared baking dish. Fill manicotti noodles with the cheese-meat mixture, being very careful because they can slice and break easily. Arrange the stuffed manicotti in a single layer in baking dish.
Spoon 1 cup of marinara sauce over manicotti (you can use the leftover marinara sauce as extra). Sprinkle the remaining 1 1/2 cups of mozzarella cheese and 1/2 cup of Parmesan cheese over manicotti.
Bake 30 -35 minutes, or until heated through and cheese is melted. Let sit for 5 minutes, serve and enjoy!