Preheat the oven to 375 degrees F. Coat a 9"x13" baking pan with cooking spray.
Spread 1 1/2 cups of the marinara sauce in an even layer in the bottom of the pan.
Place the ricotta cheese, 1 1/2 cups of mozzarella cheese, Italian seasoning, egg, salt, pepper and parmesan cheese in a bowl. Stir to combine.
Fill each shell with the ricotta mixture and place in the baking dish.
Spoon the remaining marinara sauce over the shells, then sprinkle the other 1 1/2 cups of cheese over the top.
Cover the dish with foil. Bake for 20 minutes. Uncover the pan, then bake for an additional 10 minutes or until cheese is melted and starting to brown.
Sprinkle with parsley, then serve.