Cook the pasta shells until al dente, then drain and let them cool slightly.
In a mixing bowl, combine ricotta, mozzarella, Parmesan, spinach, egg, parsley, salt, and pepper. Stir until well blended.
Preheat the oven to 375°F (190°C). Spread a layer of marinara sauce on the bottom of a baking dish.
Fill each cooked shell with the cheese mixture and arrange them in the dish.
Pour the remaining marinara sauce over the shells and sprinkle additional mozzarella on top.
Cover with foil and bake for 20 minutes, then remove the foil and bake for another 10 minutes until the cheese is golden and bubbly.
Garnish with extra parsley and serve warm.