My children think of this as comfort food. It also makes a great breakfast casserole (I always make it for the Easter breakfast potluck at church), and it can be assembled the night before. I found this recipe years ago on the Better Homes and Gardens website, and it has never failed me.
Preheat oven to 350 degrees. Grease a 9x13 pan, or line with foil and spray. Crush the tube of crackers very well.
Mix the sour cream and the cream soup together thoroughly. Add diced ham, minced onion, and cheddar. Then mix in the potatoes. (You can let them thaw a little, but they aren't as fragile when frozen, so don't let them get too soft.) Spread into prepared pan. You may have to pack it in a bit to make it fit.
Sprinkle the crushed crackers over the top, and drizzle the melted butter over the crackers. (Originally the recipe called for the crackers and the butter to be mixed together and then sprinkled over the top, but I seldom use a dish large enough to include the crackers when I melt the butter in the microwave, and I found there isn't a discernible difference between the two methods.)
Bake 45 minutes at 350 degrees F. Sprinkle on the 4 oz. mozzarella, and bake 10 more minutes. Alternatively, you can bake the casserole for 50 minutes, and sprinkle the mozzarella on before serving, letting it melt but not brown, if you don't want it to turn into a crust on the cracker crumbs. Serve hot.