1. Preheat the Oven:
   1. Set your oven to 375°F (190°C). Grease a 9×13-inch baking dish.
2. Cook the Manicotti Shells:
   1. Boil the manicotti shells according to package instructions until al dente. Drain and rinse with cold water.
3. Prepare the Filling:
   1. In a large bowl, combine the shredded turkey, ricotta cheese, Parmesan cheese, 1/2 cup mozzarella cheese, egg, garlic powder, Italian seasoning, salt, and pepper. Mix well until fully combined.
4. Stuff the Shells:
   1. Using a spoon or piping bag, carefully fill each manicotti shell with the turkey mixture. Place the stuffed shells in a single layer in the prepared baking dish.
5. Add the Sauce:
   1. In a separate bowl, mix the marinara sauce with the broth (if using) to thin it slightly. Pour the sauce evenly over the stuffed manicotti shells, ensuring they are fully covered.
6. Bake:
   1. Cover the dish with foil and bake for 25 minutes. Remove the foil, sprinkle the remaining 1/2 cup of mozzarella cheese over the top, and bake for an additional 10 minutes, or until the cheese is melted and bubbly.
7. Serve:

Garnish with fresh parsley or basil if desired. Serve warm with a side salad or garlic bread.