1. **Prepare the Filling:**

– In a large bowl, combine ricotta cheese, chopped spinach, egg, 1 cup mozzarella cheese, 1/2 cup Parmesan cheese, salt, pepper, and nutmeg (if using). Mix until well combined.

2. **Preheat the Oven:**

– Preheat your oven to 375°F (190°C).

3. **Cook the Pasta Shells:**

– Bring a large pot of salted water to a boil. Add the cannelloni shells and cook until just tender but still firm to the bite (about 4-5 minutes less than package instructions). Drain and rinse under cold water to stop the cooking process.

4. **Stuff the Shells:**

– Transfer the filling mixture to a piping bag or a plastic bag with the corner snipped off. Carefully fill each pasta shell with the ricotta-spinach mixture.

5. **Assemble the Dish:**

– Spread 1 cup of marinara sauce over the bottom of a 9×13-inch (23x33cm) baking dish. Arrange the filled pasta shells in a single layer over the sauce. Pour the remaining marinara sauce over the top, ensuring all shells are covered.

6. **Add Cheese Topping:**

– Sprinkle the remaining 1 cup of mozzarella cheese and 1/4 cup of Parmesan cheese evenly over the top.

7. **Bake:**

– Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.

8. **Serve:**

– Remove from the oven and let it sit for 5 minutes. Garnish with fresh basil leaves if desired, and serve warm.