Preheat the oven to 350°F. Cook the jumbo pasta shells according to the package directions until tender. Drain well and set them aside until cool enough to handle.

While the shells are cooking, add the Italian sausage to a medium skillet over medium heat. Cook until browned, breaking the sausage into small pieces as it cooks. Drain any excess grease if needed.

Spread enough pasta sauce over the bottom of a 9x13-inch casserole dish to lightly cover it.

In a large bowl, combine the cooked sausage, ricotta cheese, Parmesan cheese, half of the mozzarella, Italian seasoning, garlic powder, pepper, salt, and 2 cups of the pasta sauce.

Spoon the sausage and cheese mixture into each cooked shell. Arrange the filled shells in the prepared casserole dish in a single layer. Pour the remaining pasta sauce over the shells and sprinkle the remaining mozzarella cheese over the top.

Cover the casserole dish with foil and bake for 30 minutes. Remove the foil and continue baking for another 20 to 30 minutes, or until the cheese is melted and the casserole is hot and bubbly. If desired, broil for 2 to 3 minutes to lightly brown the cheese. Sprinkle with chopped fresh parsley before serving.