In a medium skillet over medium heat, cook chicken until opaque and juices run clear.
In a medium bowl, combine cooked chicken, egg, spinach, cottage cheese and Parmesan. Stuff uncooked manicotti shells with chicken mixture. Arrange shells, not touching one another, in a 9x13 inch baking dish.
In a medium bowl, combine soup, sour cream, milk and Italian seasoning and stir until smooth. Pour over shells in dish and spread to cover completely. Carefully pour boiling water around the edge of the dish. Cover tightly with foil.
Bake in preheated oven 60 minutes, or until pasta is tender. Sprinkle with Cheddar and parsley and let stand 10 minutes before serving.