Preheat the Oven: Set your oven to 375°F (190°C) to ensure it's hot and ready for baking.
Cook the Pasta: Bring a large pot of salted water to a boil. Add the large pasta shells and cook them according to the package instructions until they are al dente. Once cooked, drain the shells and give them a quick rinse with cold water to stop the cooking process. Let them cool slightly while you prepare the filling.
Prepare the Cheese Mixture: In a spacious mixing bowl, combine the ricotta cheese, 1/2 cup of the grated mozzarella cheese, Parmesan cheese, chopped spinach, minced garlic, Italian seasoning, and a pinch of salt and pepper. Stir thoroughly until the mixture is creamy and well incorporated.
Stuff the Shells: Using a spoon, carefully fill each pasta shell with the cheese mixture. Gently press the mixture into the shell to ensure it's filled well. Place the stuffed shells aside.
Layer the Baking Dish: In a 9x13-inch baking dish, spread approximately 1/2 cup of the Alfredo sauce evenly across the bottom. This will create a delicious base for the pasta.
Arrange the Stuffed Shells: Carefully lay the stuffed pasta shells in a single layer over the Alfredo sauce in the baking dish.
Add More Sauce: Pour the remaining Alfredo sauce over the stuffed shells, ensuring that each one is nicely covered to prevent drying out during baking.
Top with Cheese: Sprinkle the remaining grated mozzarella cheese evenly over the top of the shells for a delightful cheesy crust.
Bake Covered: Cover the baking dish tightly with aluminum foil and place it in the preheated oven. Bake for 25 minutes.
Finish Baking: After 25 minutes, carefully remove the foil and continue baking for an additional 10-15 minutes, or until the cheese is bubbling and has turned golden brown.
Serve and Garnish: Once baked, allow the dish to cool for a few minutes before serving. For an aromatic finish, garnish each serving with fresh basil leaves to enhance the flavor and presentation.