Preheat your oven to 375 degrees F.
In a mixing bowl, combine ricotta, mozzarella, Parmesan, spinach, egg, nutmeg, salt, and pepper. Mix until well blended.
Fill a large pot with water and bring to a boil. Cook cannelloni tubes for 2-3 minutes, then drain and let cool slightly.
Stuff each cannelloni tube with the cheese and spinach mixture using a teaspoon or a piping bag.
Spread half of the marinara sauce on the bottom of a baking dish.
Arrange the filled cannelloni in a single layer over the sauce.
Cover the cannelloni with the remaining marinara sauce and then drizzle the bechamel sauce on top.
Sprinkle additional mozzarella and Parmesan cheese over the sauces.
Bake in the preheated oven for 30-35 minutes, or until the top is golden and bubbly.
Let the cannelloni rest for 5 minutes before serving.