Thaw potatoes for approximately 10 minutes before assembling. Preheat oven to 350°F. Thoroughly grease a 9in x13in. casserole dish. Set aside.
In a large mixing bowl, stir together potatoes, cream soup, sour cream, onion, cheeses, salt and 4 tbsp melted butter. Mix well until all potatoes are covered and all ingredients are evenly distributed.
Pour potato mixture into greased casserole dish and fill, leveling the top.
In a small bowl, combine the gently crushed corn flakes with the remaining 4 tbsp melted butter. Stir until corn flakes are evenly coated.
Pour buttered corn flakes on top of casserole mixture and spread to evenly cover. Press some of the topping down slightly to avoid too many loose flakes.
Bake uncovered on 350°F for 55-60 minutes, or until top is lightly browned.Let stand for 10 minutes before serving. Refrigerate any leftovers, up to 4 days.
Notes
You can make this recipe in 2 smaller glass dishes (6 to 8 cups each) and bake for 45-50 minutes, until browned. Enjoy one right away then freeze the other for later.
To make ahead: assemble everything but the corn flake topping and cover in the refrigerator for up to 2 nights. Remove from fridge and top with corn flake mixture then bake as instructed.
To freeze: bake as directed then cool. Top with foil or parchment and cover with an airtight lid. Freeze for up to 1 month (due to dairy and sour cream). To reheat from frozen: bake on 350 degrees F, covered with foil, until heated through.