In a large mixing bowl, combine the dairy sour cream and the condensed cream of chicken soup. Stir in the frozen potatoes, cubed ham, cubed American cheese, and chopped onion, if you like.
Turn mixture into a 13x9x2-inch baking dish, spreading evenly. Combine cornflakes and the melted butter or margarine. Sprinkle over the potato mixture.
Bake in a 350 degree F oven for 30 minutes.
Sprinkle with mozzarella cheese. Return to oven and bake for 20 to 25 minutes more or until bubbly around the edges and heated through. Makes 8 to 10 servings.