Cook the Manicotti Shells. Bring a large pot of salted water to a boil. Cook the manicotti 1-2 minutes less than the package instructions (they will finish cooking in the oven). Drain and rinse with cold water to prevent sticking.
Make the Cheese Filling. In a large mixing bowl, combine ricotta, mozzarella, Parmesan, egg, garlic powder, Italian seasoning, salt, and black pepper. Mix until smooth and well combined.
Fill the Manicotti Shells. Transfer the cheese mixture to a large piping bag or zip-top bag. Cut the tip and carefully pipe the filling into each manicotti shell. Alternatively, use a spoon to stuff the shells gently.
Assemble the Dish. Preheat the oven to 375°F. Spread 1 cup of marinara sauce evenly across the bottom of a 9x13-inch baking dish. Arrange the filled manicotti shells in a single layer on top of the sauce. Pour the remaining 1 cup of marinara sauce over the shells. Sprinkle with ½ cup of shredded mozzarella cheese.
Bake to Perfection. Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for another 10 minutes, or until the cheese is bubbly and slightly golden.
Garnish and Serve. Let the manicotti cool for 5 minutes before serving. Garnish with fresh basil or parsley for added freshness. Serve with garlic bread and a crisp green salad for a complete meal!