Cook the Pasta: Preheat your oven to 375°F (190°C). Fill a large pot with water, add a generous pinch of salt, and bring it to a rolling boil. Add the jumbo pasta shells and cook until al dente, according to the package instructions, usually about 9-10 minutes. Drain the pasta and rinse with cool water to stop the cooking process. Set aside.

Prepare the Filling: In a large bowl, combine the ricotta cheese, 1 cup of shredded mozzarella, grated parmesan, chopped spinach, minced garlic, and the egg. Add the nutmeg, if using, and season with salt and pepper to taste. Mix until well combined.

Stuff the Shells: Using a spoon, carefully fill each pasta shell with the ricotta and spinach mixture. Place each filled shell on a plate and set aside.

Prepare the Baking Dish: Spread 1 cup of marinara sauce on the bottom of a 9x13-inch baking dish to create a base for the stuffed shells.

Arrange the Shells: Place the stuffed shells in the baking dish on top of the marinara sauce, open side up. Arrange them close together so they fit snugly in the dish.

Top with Sauce and Cheese: Pour the remaining 1 cup of marinara sauce over the top of the stuffed shells, making sure each shell has some sauce on it. Sprinkle extra mozzarella cheese over the top of the sauce for an extra cheesy finish.

Bake: Tightly cover the baking dish with foil and place it in the preheated oven for 20 minutes. After 20 minutes, remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.

Serve: Remove the stuffed shells from the oven and let them cool for a few minutes. Garnish with freshly chopped basil or parsley before serving.