Cook shells according to package directions. Be careful not to overcook them or they will tear easily when trying to fill them. Drain and set shells out individually on a cutting board or baking sheet so that they don’t stick together.
Preheat oven to 350°. Lightly grease two 9x13 baking dishes.
In a large bowl, combine chicken and all the rest of the ingredients except mozzarella and spaghetti sauce. Stir in 1 cup mozzarella. Spoon into shells.
Spread half of spaghetti sauce into prepared baking dishes. Arrange shells over sauce; top with remaining sauce. Cover tightly with heavy-duty aluminum foil. If you are freezing one of the 9x13 prepared pans, freeze it after this step. Include a baggy of the remaining mozzarella for when you cook it.
Bake, covered, for 40 minutes. Uncover, sprinkle with remaining 1 cup mozzarella. Bake for 5 to 10 more minutes.
