Triple Cheese Scalloped Potatos

4 pounds russet potatoes, peeled, and sliced ( about 12-13 med potatos)
1 teaspoon salt
1 stick butter
1/4 cup flour
1/4 onion, finely chopped
3 cups whole milk
3 cups extra-sharp cheddar cheese, grated
1/2 cup (packed) freshly grated Parmesan
1 cup white American cheese, cut into small cubes
1/2 teaspoon ground black pepper
2 cup crushed cornflakes cereal

1.Place the potatoes in a large pot.  Cover with water, add salt, then bring to a boil on the stovetop until tender enough to be pierced with a fork.  Drain the potatoes.  Transfer to a deep baking dish.
2.While the potatoes are cooking, heat 1/2 the stick of butter in a large saucepan.  Add the onions and saute until softened over medium heat.  Whisk in the flour to make a thick, pasty roux.  Slowly begin whisking in the milk.  Continue whisking the milk until the mixture thickens slightly.  Stir in the cheeses.  Salt & pepper to taste.

3.Pour the cheese mixture over the potatoes.  Stir the potatoes slightly until they are well coated with the cheese mixture.

4.In a small, microwave safe bowl, melt the remaining butter, then stir in the crushed cornflakes.  Sprinkle atop the potatos. 

5.Bake in an oven preheated to 350 degrees for 30-40 minutes, or until the cheese is bubbling and the cornflakes have turned golden brown.  Allow to cool slightly before serving.