Preheat the oven to 350°F (175°C).
Cook the manicotti shells according to the package directions until al dente, then drain and rinse under cold water to stop cooking.
In a mixing bowl, combine the ricotta, 1 cup of the mozzarella, the Parmesan, chopped spinach, Italian seasoning, and the egg until evenly mixed.
Fill a piping bag or use a small spoon to stuff each manicotti shell with the cheese mixture.
Spoon 1 cup of marinara sauce into the bottom of a 9×13-inch baking pan and spread into an even layer.
Arrange the stuffed manicotti in a single even layer over the sauce in the pan.
Pour the remaining marinara sauce over the manicotti and sprinkle with the remaining mozzarella.
Bake uncovered for 25–30 minutes, until the cheese is melted and bubbly.
Remove from the oven and let sit a few minutes before serving.