Step 1: Cook the Pasta : Bring a large pot of salted water to a boil. Cook jumbo pasta shells according to package instructions until al dente. Drain and rinse with cold water to prevent sticking.

Step 2: Prepare the Cheesy Filling : In a mixing bowl, combine ricotta, mozzarella, Parmesan, egg, garlic powder, Italian seasoning, salt, and pepper. Stir in spinach or mushrooms if using.

Step 3: Make the Alfredo Sauce : In a saucepan, melt butter over medium heat. Add minced garlic and sauté for 30 seconds until fragrant. Pour in heavy cream and bring to a gentle simmer. Stir in Parmesan cheese until smooth. Season with salt, pepper, and nutmeg (optional).

Step 4: Stuff the Shells : Spoon the cheese filling into each cooked pasta shell.

Step 5: Assemble the Dish : Preheat oven to 375°F (190°C). Spread a thin layer of Alfredo sauce in a 9x13 inch baking dish. Place stuffed shells in the dish in a single layer. Pour remaining Alfredo sauce over the shells. Top with shredded mozzarella cheese.

Step 6: Bake : Cover with foil and bake for 20 minutes. Remove foil and bake an additional 10 minutes until cheese is bubbly and lightly golden.