Preheat oven to 350-degrees.
Put potatoes in a large mixing bowl. Sprinkle with salt and pepper. Set aside. I can't tell you how much salt you'll need. Potatoes can be a little tricky - they often require more salt than seems reasonable. But, I'd rather have too little salt than to have too much salt. You can always add more at the table per personal preference.
Mix together the cream of chicken soup, sour cream, and melted butter.
Stir in the cheese and onions and mix well.
Pour over potatoes. Mix lightly.
Transfer to a lightly buttered 9" x 13" baking dish or a dish of equal volume.
Sprinkle cornflake crumbs (or chosen topping) over top of potato mixture. Bake, uncovered, for 1 hour until casserole is bubbly and the top is crispy.