Step 1: Cook jumbo pasta shells according to package directions until al dente. Drain, rinse with cold water to prevent sticking, and set aside to cool.
Step 2: In a large bowl, combine the shredded or diced cooked chicken, ricotta cheese, 1 cup of shredded mozzarella cheese, 1/2 cup of grated Parmesan cheese, the large egg, garlic powder, onion powder, Italian seasoning, salt, and black pepper. Mix until all ingredients are well combined.
Step 3: In a medium saucepan, melt the unsalted butter over medium heat. Add the minced garlic and sauté for 1 minute until fragrant.
Step 4: Slowly whisk in the heavy cream. Bring the mixture to a gentle simmer, then reduce the heat to low.
Step 5: Stir in the remaining 1/2 cup of grated Parmesan cheese and a pinch of nutmeg. Continue to stir until the sauce is smooth and slightly thickened. Season with additional salt and pepper to taste.
Step 6: Preheat your oven to 375°F (190°C). Pour about half of the prepared Alfredo sauce into the bottom of a 9x13 inch baking dish, spreading it evenly.
Step 7: Carefully stuff each cooled pasta shell with the chicken and cheese mixture, using approximately 1-2 tablespoons per shell.
Step 8: Arrange the stuffed shells in a single layer in the prepared baking dish.
Step 9: Pour the remaining Alfredo sauce over the stuffed shells, ensuring they are well coated. Sprinkle with the remaining 1 cup of shredded mozzarella cheese.
Step 10: Cover the baking dish tightly with aluminum foil and bake for 20 minutes.
Step 11: Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden brown.
Step 12: Garnish with fresh chopped parsley before serving, if desired. Serve hot and enjoy!