Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with non-stick cooking spray or a light drizzle of olive oil.

Heat a splash of water in a large skillet over medium heat, add the baby spinach, and cook for 2 to 3 minutes until completely wilted. Transfer the spinach to a fine-mesh strainer, let it cool slightly, and squeeze out as much excess moisture as possible before roughly chopping it.

In a medium mixing bowl, combine the chopped spinach, ricotta cheese, half of the shredded mozzarella, half of the Parmesan, the egg, minced garlic, and grated nutmeg. Season generously with salt and black pepper, stirring well until the mixture is smooth and fully incorporated.

Spoon about 1 cup of the marinara sauce into the bottom of the prepared baking dish, spreading it into an even layer to prevent the pasta from sticking.

Transfer the spinach and ricotta mixture into a piping bag or a large zip-top bag with the corner snipped off. Carefully pipe the filling into each dry cannelloni tube until full, then arrange them in a single layer in the baking dish.

Pour the remaining marinara sauce evenly over the stuffed cannelloni, ensuring all the pasta tubes are completely covered so they cook evenly without drying out.

Sprinkle the remaining mozzarella and Parmesan cheeses over the top of the sauce-covered pasta. Cover the baking dish tightly with aluminum foil and bake for 35 minutes.

Remove the foil and bake for an additional 5 to 10 minutes, or until the cheese is melted, golden brown, and bubbling at the edges. Let the dish rest for 5 minutes before serving with a garnish of fresh basil leaves.