1. To make the béchamel sauce: Set a medium pot over medium heat and add butter and flour. Stir constantly for four to five minutes until mixture is a light golden brown. Add salt, pepper, and nutmeg. Continue stirring and slowly add in milk and heavy cream, a quarter cup at a time. Use a whisk to break up any lumps. Keep stirring until tiny bubbles form around the edge of the pot. If desired, add Parmesan cheese and stir for an additional minute. Remove from heat.
2. Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).
3. Grease a nine- by 13-inch (20- by 30-centimeter) baking dish. Pour a third of the sauce on the bottom of the dish.
4. Place all the filling ingredients in a large bowl and mix well to combine. Put mixture into a piping bag or ziplock bag. Snip off the corner of the bag and pipe filling into the shells.
5. Place shells in a single layer onto the sauce in the dish. Cover all the filled shells with the remaining béchamel. Sprinkle the top with the two cups of mozzarella and place tiny pieces of butter over the cheese.
6. Bake uncovered for about 35 minutes until the top is golden and bubbly. Allow to cool for eight minutes before serving.
For an alternative to the shells, you can use cannelloni or lasagna noodles. To save yourself the step of parboiling, use the oven-ready pasta.