Preheat the oven to 375°F (190°C) and spread 1/4 cup of marinara sauce on the bottom of a 9x13 inch baking dish.

In a medium mixing bowl, combine thawed spinach, ricotta, 3/4 cup shredded mozzarella, 1/4 cup grated Parmesan, egg, minced garlic, 1/2 teaspoon salt, 1/4 teaspoon black pepper, and nutmeg; mix gently until just combined.

Bring a large pot of salted water to a rolling boil, add cannelloni tubes, and cook 8–10 minutes until just tender; drain and rinse under cold water, then spread on a towel to cool.

In a small saucepan over medium heat, melt butter, whisk in flour and cook for 1 minute, then slowly whisk in milk until the béchamel sauce thickens, about 3–4 minutes; season with salt and pepper.

Using a small spoon or piping bag, fill each pasta tube with about 2 tablespoons of the ricotta-spinach mixture; place seam-side down in the prepared baking dish.

Pour remaining marinara sauce evenly over the filled tubes, then drizzle the béchamel sauce on top.

Sprinkle the remaining 1/2 cup mozzarella and 1/4 cup Parmesan cheese over the dish.

Bake uncovered at 375°F (190°C) for 35–40 minutes until the top is golden brown and the sauce is bubbling.

Remove from the oven and let the cannelloni rest for 5–10 minutes before serving.