Preheat your oven to 375°F (190°C). While waiting, squeeze all excess moisture from the thawed spinach - this is crucial for a perfect filling.
In a large bowl, combine the drained spinach, ricotta, Parmesan, eggs, garlic, nutmeg, salt, and pepper. Mix until well combined but don't overmix.
Spread 1 cup of marinara sauce in the bottom of a 9x13 inch baking dish.
Fill each cannelloni tube with the spinach mixture. Pro tip: Use a piping bag or zip-top bag with the corner cut off for easier filling.
Arrange the filled tubes in the baking dish, making sure they don't overlap.
Pour the remaining marinara sauce over the cannelloni, ensuring all pasta is covered.
Sprinkle mozzarella cheese evenly over the top.
Cover with foil and bake for 35 minutes. Remove foil and bake for an additional 10-15 minutes until golden and bubbly.