Cook manicotti shells according to package instructions until al dente. Drain and let cool slightly.
In a bowl, mix ricotta, mozzarella, Parmesan, egg, salt, pepper, and Italian seasoning until smooth.
Fill each shell carefully using a piping bag or spoon.
Spread marinara sauce at the bottom of a baking dish.
Place stuffed manicotti in the dish and cover with remaining sauce.
Sprinkle extra mozzarella and Parmesan on top.
Cover with foil and bake at 375°F (190°C) for 25-30 minutes.
Remove foil and bake for another 10 minutes until bubbly.
Let rest for 5-10 minutes before serving.