01 -  Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish. 
 02 -  Boil jumbo pasta shells in a large pot of salted water until al dente, about 8 to 9 minutes. Drain and let cool slightly. 
 03 -  Combine ricotta, mozzarella, Parmesan, spinach, egg, salt, black pepper, and nutmeg (if using) in a large bowl until well blended. 
 04 -  Melt unsalted butter in a medium saucepan over medium heat. Sauté minced garlic for 1 minute until fragrant. Add heavy cream and bring to a gentle simmer. Whisk in Parmesan cheese, salt, and black pepper, stirring until smooth and slightly thickened, about 3 to 4 minutes. Remove from heat. 
 05 -  Spread 1/2 cup of Alfredo sauce onto the bottom of the greased baking dish. 
 06 -  Fill each cooked shell with approximately 2 tablespoons of the ricotta-spinach mixture. Arrange the filled shells in the baking dish. 
 07 -  Pour the remaining Alfredo sauce evenly over the stuffed shells. Sprinkle with mozzarella and Parmesan cheese. 
 08 -  Cover the baking dish tightly with aluminum foil and bake for 20 minutes. Remove foil and continue baking for 10 to 15 minutes, until the dish is bubbly and golden. 
 09 -  Allow the baked shells to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.