1. Preheat oven to 375°F (190°C) and grease a 9×13-inch baking dish.

2. Cook pasta shells in salted boiling water until al dente, drain, rinse under cold water, and set aside.

3. In a bowl, combine shredded chicken, ricotta, half of the mozzarella, Parmesan, spinach, garlic, salt, and pepper.

4. Make the white sauce by melting butter in a saucepan over medium heat, whisking in flour to form a roux, then gradually whisking in milk until smooth and thickened.

5. Season the sauce with salt, pepper, and a pinch of nutmeg, then stir in remaining mozzarella until melted.

6. Spread a thin layer of sauce in the bottom of the baking dish.

7. Stuff each shell with the chicken and cheese mixture and place seam-side up in the dish.

8. Pour half of the white sauce over the shells, then sprinkle with remaining cheese and drizzle with the rest of the sauce.

9. Cover with foil and bake for 20 minutes, remove foil and bake an additional 15 minutes until bubbly and golden.

10. Let rest 5 minutes, garnish with parsley, and serve hot.