Preheat the oven to 350°F (175°C).
Cook the manicotti shells according to 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, Parmesan, chopped spinach, Italian seasoning, and the egg until evenly mixed.
Spoon or pipe the cheese mixture into each manicotti shell, filling them evenly but without overstuffing.
Spoon 1 cup of marinara sauce into the bottom of a 9×13-inch baking dish and spread into an even layer.
Arrange the stuffed manicotti in a single layer over the sauce, then pour the remaining marinara over the pasta and sprinkle with the remaining 2 cups of mozzarella.
Bake uncovered for 25–30 minutes, until the cheese is melted and bubbly.
Let the dish rest a few minutes before serving.