Preheat the oven to 350°F (175°C).
Cook the manicotti shells according to the package directions until al dente, then drain and rinse with cold water to stop cooking; set aside to cool slightly.
In a medium bowl, combine the ricotta, 1 cup of the mozzarella, Parmesan, chopped spinach, Italian seasoning, and the egg until evenly mixed.
Fill a piping bag or use a spoon to stuff each manicotti shell with the cheese mixture, taking care not to overfill.
Spoon 1 cup of marinara sauce into the bottom of a 9x13-inch baking dish and spread into an even layer.
Arrange the stuffed manicotti in a single layer over the sauce, cover with the remaining marinara, then sprinkle the remaining mozzarella evenly over the top.
Bake uncovered for 25–30 minutes, until the cheese is melted and bubbly. Let rest a few minutes before serving.
