Bring a large pot of salted water to a boil, then add the large pasta shells. Cook until just al dente, about 8 minutes, until they are tender but still hold their shape. Drain and set aside to cool slightly.
In a mixing bowl, combine ricotta cheese, shredded mozzarella, chopped spinach, and a pinch of nutmeg. Mix until well blended and creamy, with the spinach evenly distributed.
Using a spoon or piping bag, carefully stuff each shell with the cheese mixture, overflowing slightly for a generous filling. Place the stuffed shells in a baking dish, close together but not overlapping too tightly.
Pour the marinara sauce evenly over the stuffed shells, making sure each shell is coated. Sprinkle grated Parmesan cheese over the top for a crispy finish.
Preheat your oven to 180°C (350°F). Bake the assembled shells uncovered for 25 to 30 minutes, until the sauce is bubbling and the Parmesan has formed a golden crust.
Once baked, remove the dish from the oven and let it rest for about 5 minutes. The cheese will set slightly, and the aroma of baked cheese and tomato will fill your kitchen.
Serve hot, with a sprinkle of extra Parmesan if desired, and enjoy the gooey, crispy, and comforting flavors of these creepy stuffed shells.
