This is one of my most requested recipes. I splurge when I make these and use heavy cream, but milk works just fine too. This is easily adjustable to make more or less shells, and to season per personal preference. (Forgive the bad picture, it is old and taken when I didn't know how to photograph food!)
Preheat oven to 350F. Spray a baking/casserole dish with cooking spray. Cook shells according to package directions.
Mix chopped chicken, egg, ricotta, parsley, 1 1/2 cups mozzarella and 1/2 cup parmesan in bowl and set aside.
Make white sauce: Melt butter in saucepan, add flour. Cook a little, to get rid of the flour-y taste, but don't let butter brown. Whisk in cream, salt and pepper. Once thickened stir in parmesan cheese.
Spoon a little bit of sauce in each cooled shell and fill each with chicken mixture.
Spread half of the marinara on the bottom of your pan. Arrange shells in the pan and pour white sauce over the shells.
Drizzle the rest of the marinara over the top and sprinkle remaining mozzarella over all.
Cover and bake about 40 minutes until hot through out. Uncover and bake another 15 minutes until golden and bubbly.