Combine the sour cream, chicken and mushroom soup, cheddar cheese, scallions, salt, and pepper in a large bowl and mix until well combined. Stir in the grated potatoes.
Pour the potato mixture into a casserole dish.
Combine the corn flakes, melted butter, Parmesan, and a pinch of salt and pepper in a medium bowl, then sprinkle on top of the potato mixture. Bake for 1 hour, or until the casserole is bubbling and the potatoes are cooked through.