In a large pot of boiling water, cook shells until al dente, about 2 minutes less than directions on package call for. Place on greased baking sheet to cool down
In a mixing bowl, mix the chicken, ricotta cheese, 1 cup of mozzarella cheese, Parmesan cheese, salt, Italian seasoning and pepper until well combined
Preheat oven to 350 degrees, and spray a 13 x 9 baking dish with non stick spray
Gently stuff about 2 tablespoons of the chicken mixture into each shell. Place stuffed side up in baking dish
In large skillet, melt the butter over medium heat. Stir in heavy cream, and increase heat to medium high
Stir in nutmeg. When mixture starts to bubble, reduce heat to medium, and stir in Parmesan cheese, 1 cup at a time, stirring or whisking continuously until smooth and starting to thicken. Sauce should coat the back of the spoon
Remove from heat, and pour over stuffed shells, making sure sauce covers the bottom of the pan
Sprinkle remaining cup of mozzarella cheese over shells
Lightly spray a large sheet of aluminum foil with cooking spray, and place loosely over pan, sprayed side down
Place into preheated oven, and bake for 15 minutes. Then, remove foil, and bake for another 15 minutes or until cheese is melted and sauce is bubbling
Allow to cool for about 10 minutes so that sauce thickens more, then serve